Frequently Asked Questions

Environment

Are carp a symptom or a cause of environmental damage?

Non-scientific

Scientific

The answer is both. Australian rivers experience many environmental pressures – carp are just one. Separating carp impacts from other sources of environmental stress is difficult for two main reasons. Firstly, carp thrive in rivers that are already degraded, and tend to intensify the impacts of other environmental pressures. Secondly, the footprint carp leave on the environment extends over large areas, and can result in sudden shifts between different ecosystem states (e.g. clear vs. muddy water) - rather than varying predictably in direct relation to carp numbers.

Research on carp impacts through the 1990s provided evidence that carp really do damage river ecosystems. Carp muddy waters, increase nutrient levels (promoting blue-green algae blooms), and reduce abundance of water plants (macrophytes), invertebrates (e.g. aquatic insects and crustaceans), and some fish. Carp also increased water turbidity (muddiness) in 91% of studies, reduced invertebrates in 94%, and reduced water plants in 96% of studies.

Carp impacts also tend to be interlinked and cumulative. Their bottom-feeding behaviour reduces water clarity, limiting sunlight to water plants. This, in turn, reduces habitat and/or food for invertebrates, native fish and waterbirds. The overall effect of these impacts is to shift ecosystems from a predominantly clear-water state to a murky, nutrient-rich state (‘eutrophication’). Once an ecosystem shifts in this direction, it can be difficult to reverse, meaning that the river will remain muddy for some time, even as carp numbers fluctuate in locations within the system.

Carp are both a cause and a symptom of environmental damage in Australian waterways. However, separating impacts caused by carp from other stressors is difficult. Carp impacts occur across a range of spatial and temporal scales, interact with other stressors and have cumulative, emergent properties. Understanding carp impacts and quantifying them scientifically is difficult, requiring well-planned, multi-year experiments in different types of systems.

Nonetheless, increased research on carp impacts through the 1990s provided evidence that carp clearly do damage river ecosystems. This research included systematic reviews and meta-analyses, which combine and analyse data from numerous studies on a particular topic, as well as large-scale experiments. These studies prove that carp muddy waters, increase nutrient levels (thereby promoting blue-green algae blooms), and reduce abundance of water plants (macrophytes), invertebrates (e.g. aquatic insects and crustaceans), and some fish species (Vilizzi et al., 2014, 2015; Weber and Brown, 2009). For example, Weber and Brown (2009) found that carp increased water turbidity (muddiness) in 91% of surveyed studies, reduced invertebrates in 94%, and reduced macrophytes in 96% of surveyed studies. A more recent meta-analysis supported these results, finding strong evidence for carp impacts on all the same ecosystem components (Vilizzi et al., 2015, Table 1).

Carp impacts also tend to be interlinked. Adult carp feed by sucking sediments from the river bed, filtering out food items and puffing the remaining mud into the water column. This feeding style reduces water clarity, which limits the sunlight penetrating down to macrophytes on the river bed. Fewer macrophytes mean less habitat for invertebrates and native fish. Smaller carp compete with native fish for planktonic food sources. The cumulative effect of these impacts is to shift ecosystems from a predominantly clear-water state (‘oligotrophic’), to a murky, nutrient-rich state (‘eutrophic’). Shifts from one state to another are often termed ‘phase shifts’ in ecology. Once an ecosystem has shifted to a new phase, reversing the change is usually difficult, meaning that the river will remain muddy for some time, even as carp densities fluctuate in various locations within the system.

Australian studies have also demonstrated carp impacts in Australian waterways. King et al. (1997) examined effects of carp density on turbidity, phytoplankton (microscopic algae), and nutrients in two billabongs. The study found clear evidence of carp impacts, with the authors reporting that, "in these natural billabongs, high standing stocks of carp caused increases in turbidity and more intense algal blooms."

Finally, a recent study on the lower Murray River used an experimental design with considerable power to detect carp effects, demonstrating that carp can drive phase shifts from clear to dirty water states, with the latter characterised by poor populations of macrophytes and aquatic invertebrates (Vilizzi et al. 2014). Importantly, this study also indicated that carp may cause environmental damage at lower densities than previously considered.

In summary, acknowledging that Australian rivers face degradation from many sources should not preclude carp control. Nor should action to reduce carp impacts diminish efforts to restore waterway health via other means. Rather, an integrated carp control program could support broader river rehabilitation programs, including habitat restoration and water-quality remediation.

Weber, M. J. and Brown, M. L. (2009). Effects of common carp on aquatic ecosystems 80 years after “Carp as a Dominant”. Reviews in Fisheries Science 17, 524 – 537.

How did carp get here?

Non-scientific

Scientific

Slow beginnings

Carp had a slow start in Australia, which is surprising given their wide distribution and high numbers today. During the mid-1800s, attempts were made to introduce carp in Victoria, New South Wales, and Tasmania. None of these early introductions appear to have resulted in large, self-sustaining populations. Similarly, two attempted introductions in Victoria during the 1870s failed to become established. Localised populations of carp became established in New South Wales around 1907-1910, following two introductions comprising a total of about 15 carp into an inlet pond above Prospect Reservoir. This strain (genetic variant) of carp, known as the ‘Prospect Strain’ probably maintains a locally-restricted distribution in the area.

The Boolarra strain appears

Early introductions were followed by other releases, some involving up to 50,000 fish, through the 1930s-1950s. However, carp numbers seem to have remained relatively low through to the early 1950s. This pattern of limited geographic spread and relatively low abundance changed when carp (produced by Boolarra Fish Farms Pty. Ltd. in Gippsland during the late 1950s) were introduced into a reservoir at Morwell, Victoria in the 1960s. Rapid spread of these ‘Boolarra Strain’ carp within Victoria followed, and by 1962 a Victorian state government inquiry had determined that carp should be eradicated.

We’ve got a problem: expansion to the present day

‘Boolarra Strain’ carp had gained access to the Murray River by the mid-late 1960s, despite eradication attempts using poisons by the Victorian Department of Fisheries and Wildlife. Extensive flooding in 1974-75, and again during the early 1990s, facilitated the species’ spread. People also aided the spread of carp, through deliberate translocation, undetected presence of carp among stocked native fish, and the use of small carp as live bait for predatory fish. The latter is thought to be the primary mechanism explaining the presence of carp in several Tasmanian lakes, and in NSW coastal river systems. Ornamental carp (also known as Koi) continue to be released by the public.

See the non-scientific answer to this question.

Why are carp such a problem in Australia?

Non-scientific

Scientific

There are two main reasons why carp have become a dominant pest in Australia. The first relates to their biology: carp can tolerate a wide variety of environmental conditions, have a broad diet, grow rapidly, mature early, can produce large numbers of eggs, are strong swimmers, good jumpers, and do well in ecosystems that are modified by humans. Carp also spawn earlier than many Australian native species, which means that their juveniles have access to food and other resources before many native fish species.

Environmental conditions at the time carp began dominating Australian waterways is also an important factor. The initial explosion of carp numbers in Australia in the 60‘s-70’s occurred during a ‘perfect storm’ of sorts. Many native fish species had experienced significant declines in numbers due to historically high commercial fishing pressure, widespread reduction in habitat, extensive construction of dams, weirs and other barriers to their migration, and declines in water quality due to widespread poor land use and urbanisation. These elements combined to provide the ideal conditions for a successful invader such as carp to flourish.

Koehn (2012) provides a detailed summary of the factors that are likely to have led to the success of carp in Australia. To summarise, carp possess many of the characteristics of a successful invader. Specifically, they can tolerate a wide variety of environmental conditions, have a broad diet, grow rapidly, mature early, are highly fecund, are highly dispersive, and do well in systems that are modified by humans (Gehrke 1997) (see Table 1 below from Koehn 2004).

The environmental conditions that were prevalent during the 1960’s-1970’s when carp numbers dramatically increased in Australia were also relevant. There was likely to be particularly low predatory pressure on carp during this period as a result of high commercial fishing pressure, widespread reduction in habitat, prolific construction of dams, weirs and other barriers to their migration (Koehn 2001), and water quality was generally poor due to widespread inappropriate landuse practices (Koehn 2004). Coupled with the rapid growth rate of carp and large size when mature, this is likely to have enabled a large cohort to overwhelm predatory pressure and rapidly attain a size that precluded predation.

References

Gehrke PC (1997). Differences in composition and structure of fish communities associated with flow regulation in New South Wales. In: Harris JH and Gehrke PC (Eds), Fish and Rivers in Stress: the NSW Rivers Survey. NSW Fisheries Office of Conservation and Cooperative Research Centre for Freshwater Ecology, Cronulla and Canberra. Pp 169–200.

Koehn J (2001). The impacts of weirs on fish. In: The Proceedings of The Way Forward on Weirs. Presented on 18-19th August 2000, at the Centenary Lecture Theatre, Royal North Shore Hospital, St Leonards, NSW. Inland Rivers Network, Sydney. Pp 59–66.

How many carp are there in Australia?

Non-scientific

Scientific

Fisheries scientists and managers tend to talk about how much (biomass or density) rather than how many (abundance). This is in part because biomass and density - which estimate total weight of a given organism in a certain area at a given time - is generally more useful as it relates to the amount of energy available. For a pest fish species such as carp, which quickly reach a size too large for most predators, and estimate of biomass can help reflect the amount of energy locked up in carp populations, and therefore unavailable to other species in the food web.

Carp have become the dominant freshwater fish in south-east Australia, comprising up to 80% of the fish biomass in many areas, resulting in biomasses as high as 3144 kg/ha and densities of up to 1000 individuals/ha in some parts of the Murray-Darling Basin.

To date, estimates of carp biomass/density in Australia have largely been at local or regional scales, and can vary widely. Research being conducted as part of the National Carp Control Plan will provide the most accurate and comprehensive estimate of carp biomass in Australia to date. This estimate is vital to informing clean up strategies and the modelling of possible release scenarios to deliver optimal carp control outcomes through biocontrol.

Fisheries scientists and managers tend to talk about how much (biomass or density) rather than how many (abundance). This is in part because absolute abundance is hard to measure in fish, but also because biomass and density, which estimate total weight of a given organism in a certain area at a given time is generally more useful as it relates to the amount of energy available to the next trophic level. Further, for a pest fish species such as carp that quickly reach a size too large for most predators (Koehn, 2004), estimation of biomass can help reflect the amount of energy locked up and unavailable to other species.

Carp have become the dominant freshwater fish in south-east Australia (Koehn, 2004), comprising a significant proportion of the fish biomass in many areas, resulting in biomasses as high as 3144 kg/ha and densities of up to 1000 individuals/ha in some parts of the MDB (Harris and Gehrke, 1997).

To date, estimates of carp biomass/density in Australia have largely been at local or regional scales, and vary from 190kg/ha in Moira Lake (Brown et al., 2003); to between 150-690kg/ha in a range of billabongs (Hume et al., 1983); 690 kg/ha in the Bogan River (Reid and Harris, 1997); 619 kg/ha in the Campaspe irrigation channels (Brown et al., 2003), and 10-40 kg/ha in the Logan/Albert system (Norris et al 2011).

Estimation of biomass or density at larger spatial scales bring greater uncertainty. However, estimates of carp biomass and density will be important for the NCCP, to inform mathematical modelling of virus spread, and development of strategies to clean up fish after possible virus release.

For this reason, it is proposed to improve estimates of carp biomass and density as a deliverable of the NCCP research program.

What carp control measures have been undertaken and why haven’t they worked?

Non-scientific

Scientific

Commercial carp fishing fills niche markets for human consumption, fish leather, aquaculture feedstock, bait and fertiliser. Local consumer demand for carp is limited to 50-60 tonnes per year at present. Demand from these niche markets is not enough to have any significant reduction in the current carp population.

Manual carp removal, including trapping and controlling access to breeding grounds, has seen some success in Tasmania's Lake Crescent and Lake Sorell. Lake Crescent was declared free of carp in 2007 after 12 years of manual removal work. Carp removal work is continuing in Lake Sorell. The cost of the program has been about $11 million.

Earlier research programs have explored ways to genetically alter fish to produce offspring of only a single sex. This approach does not kill affected fish, but merely pushes a population to extinction by reducing breeding opportunities. As carp have a lifespan of 35 years, it would take more than a century using this approach alone to significantly reduce the population. Both approaches are being investigated as potential long-term control measures in combination with the carp virus.

Various methods have been trialled to control carp, or reduce their impacts in Australia over the years. Primarily these have involved physical removal (e.g. netting, angling, trapping) or poisoning. All of these methods have advantages and disadvantages relating to their effectiveness, ease of use, size specificity (some remove only adult carp), impacts on non-target organisms, and cost. Methods employed for controlling carp and their impacts are summarised within Table 1, and are further discussed below.

Human resource intensive; however, events are often independently organised by community groups.

High interest from community groups in conducting these types of events. Increases community awareness.Ineffective in reducing population numbers or removing residual carp from waterways.

Commercial harvesting (hauling/netting/trapping)

Potential to remove large quantities of carp quickly in specific locations.

Market forces limit long-term effect on population.

Dependent on mesh size.

Some bycatch of non-target species.

Self-funding, but only if carp populations and market price allow for a viable, self-sustaining industry. Otherwise fee-for-service.

Requires consistent supply of large quantities for market to remain viable.Market returns justify effort only under specific conditions (high carp biomass, proximity to markets, minimal obstructions such as snags).Currently low viability because of low market price and high costs of fishing.Not viable for removal of residual carp populations in connected waterways.Judas-carp technique (where males are radio-tagged and act as 'tracker fish') may enhance effectiveness and efficiency of commercial harvest by targeting spawning or winter aggregations; this would require commitment to a long-term control program.

Rotenone

Potential to kill discrete populations of carp quickly.

Not size specific.

Broad-scale application kills virtually all non-target species as well as carp.Rotenone baits have been trialled but tended to be rejected by carp; the rotenone may also leach out and thus affect non-target species.

Human resource intensive (planning, application and removal/disposal of large quantities of carp).Moderate costs.

Illegal to use except under and in accordance with Australia Pesticides and Veterinary Medicines Authority (APVMA) permit.May be feasible to eradicate small, discrete populations under specific circumstances (e.g. new populations) where benefits clearly outweigh harm to native species. Not suitable for broad-scale use because of impacts on non-target organisms.Use of baits is not currently feasible without further improvement.

Fishway carp separation cages

Potential to remove large quantities of carp and, in some circumstances, eliminate carp from stretches upstream of cages.Effectiveness depends on the proportion of the carp population that is static vs. migrating. This is unknown for many sites.

Highly feasible where suited to existing fishways, or where new fishways are being designed. Lack of carp-disposal options may limit feasibility at some sites. Fish composting technology may be an effective utilisation and disposal method (where feasible and based on resources available from local agencies/groups).

Already installed at many sites in the Murray-Darling Basin. For maximum effectiveness, requires ecological research to identify recruitment areas for carp and native species. 'Finger traps' may be more effective but are still at prototype stage.

Sex biasing technology

In theory could eventually provide total eradication, but technically difficult, and effects would not be seen for up to 100 years due to long generation time of carp. Effectiveness would depend on many factors, including: heritability; fitness of modified fish; size of carp population at time of release; and number of modified fish released.

Not size specific.

Not size specific.

Very expensive technology, still under development, total costs unknown.

Unclear. Still many technical hurdles to overcome before ready for laboratory or field trials. Would initially require stocking of large numbers of fish carrying sex biasing construct.Would require integrated implementation with other initiatives.Risk of public non-acceptance of intentional release of modified pests into natural environment. Requires extensive public consultation.

Not size specific, although juveniles believed to be more susceptible.

Species-specific (McColl, In Prep), but mass carp mortalities post release could have water quality impacts detrimental to native species.

Unknown; still under investigation.

Strong temperature relationship may impact effectiveness at low or high temperatures. Hybridisation between goldfish and carp could reduce effectiveness.
Risk of public non-acceptance of intentional release of biocontrol agent.

Recreational fishing events are popular within the Australian community, and such events can enable quantities of carp biomass to be removed from an area, although research suggests that this does not result in a lasting reduction in carp numbers (Gehrke, 2010). For example, the mean estimated population reduction by anglers in the Goondiwindi Carp Cull was reported to be 0.5% compared to 13.4% for electrofishing (Norris et al., 2013). Similarly in 2008, anglers in the Goondiwindi Carp Cull removed 40 carp from lagoon habitats in south-eastern Queensland, equivalent to 1.9% of the estimated population, and much lower than the catches provided by other methods (Gehrke, 2010).

Brown and Walker (2004) demonstrate that unless carp populations can be a reduced by a large percentage, physical removal is unlikely to offer an effective method for carp control. Similarly, Gehrke (2010) suggests that low-cost carp angling events provide an effective method for promoting community awareness of issues surrounding carp in the Murray-Darling Basin, but their effectiveness in reducing carp populations and environmental impacts is low (Norris et al., 2013).

Commercial harvesting

Graham et al. (2005) summarise predominant commercial fishing methods used for carp, which include electrofishing, hauling, trapping, mesh netting and angling. The limited acceptance of carp for human consumption in Australia limits its market value, with most being sold to produce low-value fishmeal, fish oil, pet food, fertiliser and stock feed. Wilson (1998), cited in Graham et al. (2005), suggested that at the time of writing, fishers needed to catch 5 – 6 tonnes of carp per week (at 80 cents/kg) to make an economic return. This means that the fishery is generally only viable under conditions that allow the removal of large volumes of carp at minimal cost. Another factor limiting the effectiveness of commercial fishing in controlling carp abundance is the increasing cost of production as biomass is reduced.

Electrofishing

Whilst electro-fishing is effective for carp removal in areas of high density, it is less effective in deep water, high turbidity or flow, and can be expensive both in terms of capital and labour costs. Whilst non-target species are also stunned during electrofishing, they generally quickly recover, and mortality levels are normally low under appropriate operating circumstances. Electrofishing is generally not widely used as a commercial fishing method for carp in Australia, but Graham et al. (2005) report that supplies of carp to a processing factory at Sale are regularly supplemented with electro-fished carp from tributaries to the Gippsland Lakes, particularly during drought conditions when carp retreat into the rivers as the lakes become more saline.

Seining

A seine net is a large sock-shaped net with a pair of long hauling lines attached to each side of the open end. The net (including ropes) is ‘shot’ from the boat around a concentration of fish and then hauled back to the boat or shore by drawing on the lines. In this way, the fish are progressively corralled into the back of the net, or cod end. Seining is one of the more effective methods for catching large quantities of carp (Bajer et al., 2011). Catch records from 2001/02 show that approximately 15 t were caught by drag net from Lake Brewster, after carp were attracted to the hauling area with berley, and up to 1000 t of carp are harvested annually from Lake Wellington, mostly by seine (Graham et al., 2005). Application of this method is limited to shallow lakes or dams where the substrate is clear of obstructions or where the bottom is relatively smooth, firm, and clear of snags (Graham et al., 2005). Most natural waterways are unsuited to seining as lakes and riverbeds are normally littered with woody debris and other snags (Graham et al., 2005).

Trapping

Unbaited drum nets were widely used to target native fish prior to discontinuation of commercial fishing in the Murray-Darling Basin in 2003. Larger baited rectangular traps have also been shown to be effective for carp but require easy access to the water. Baited traps are most effective when set downstream in flowing waterways, and when fitted with netting wings to one or both banks to guide carp into the trap. Non-target impacts of trapping can be significant, particularly for air-breathing vertebrates, if not fitted with an escape device or accessible air space.

Mesh-netting

Mesh-netting was historically the dominant method used for harvesting native fish, in the Murray-Darling. Captured fish can be damaged through scale loss and meshing injuries, and air breathing vertebrates can also become entangled. However, Graham et al. (2005) report that setting mesh nets in shallow water and frightening fish towards the net can effectively enable carp to be targeted whilst having minimal impact on non-target species.

Judas carp

The Judas carp technique (wherein males are radio-tagged to then enable the school of fish they associate with to be targeted as they re-integrate) may enhance effectiveness and efficiency of commercial harvest by targeting spawning or winter aggregations which contain populations of sexually mature carp (Gilligan et al., 2010). This method has been trialled in Lake Cargelligo in the lower Lachlan catchment and in Tasmania with some success, however is most useful in areas of low carp abundance (Bajer et al., 2011).

Rotenone

There are no fish poisons, or piscicides available that are specific to common carp, and no chemicals are fully registered as piscicides in Australia. Rotenone is the only chemical currently legal to use in Australia to control any pest fish, and it is occasionally used for this purpose (Rayner and Creese, 2006). Rotenone interrupts cellular respiration in gill-breathing animals by blocking the transfer of electrons in the mitochondria. Acute exposure to toxic levels reduces cellular uptake of blood oxygen, resulting in increased cellular anaerobic metabolism and associated production of lactic acid causes blood acidosis (Fajt and Grizzle, 1998).

Historically, Australian states and territories have applied for a ‘minor use’ permit to be able to use chemicals such as rotenone for a specified time and under permit conditions, including that there:

is a high probability of successfully eradicating the pest fish, with a low chance of immigration or recolonization;

has been a review of environmental factors that identified benefits outweigh impacts on native species;

is no risk to the health of humans, stock or domestic animals through direct contact or contaminated drinking water; and,

is generally strong public and political support for the operation.

Researchers have attempted to develop a carp-specific targeting method using rotenone, through integrating it into floating pellet baits. This method was found to be unsuccessful due to low buoyancy and palatability (Gehrke, 2003). Further development and testing would be required (and a separate APVMA permit approved) before rotenone baits could be utilised to target carp populations. Although it is a potential option to eradicate discrete new populations of carp in some limited circumstances, rotenone is not appropriate for use to control carp on a large scale.

Fishway carp separation cages

Carp separation traps that exploit the species’ jumping behaviour have been implemented at numerous locations throughout the Murray-Darling Basin. Trials with these devices on the Murray River revealed that these traps were effective in catching carp and passing native fish, with 88.8% of carp caught, 99.9% of native species passed, and catches of up to 5 t per day in some instances. It is clear that their effectiveness can be variable (Table 2), with catch per unit effort across seven installations reported to be 14.7kg per week, or 1.4 carp per day (pers. comm. M. Gordos). For this reason the NSW Department of Primary Industries no longer recommends the installation of carp separation cages at remote, un-manned locations.

Carp exclusion devices can prevent access of mature carp to wetlands or other spawning grounds, having potential to substantially reduce carp populations at a localised scale if breeding hotspots are targeted. However such methods are size specific, generally excluding only larger carp, and may affect native fish recruitment by also excluding native species from spawning grounds.

Exclusion devices are relatively inexpensive and are already installed at many sites in the Murray-Darling Basin, however require supporting infrastructure and ongoing maintenance. For maximum effectiveness exclusion devices require ecological research to identify recruitment areas for carp and native species. This information will assist in determining appropriate deployment locations. 'Finger traps' may be a more effective technique, though are still at prototype stage.

Sex biasing approaches, including daughterless carp technology

In theory sex biasing approaches including the species specific daughterless carp technology could eventually provide total eradication of common carp, however these methods have never been proven in carp, and many have never been proven in any fish species.

The effectiveness of any genetic sex biasing approach including daughterless carp technology would depend on many factors, including: heritability; fitness of modified fish; size of carp population at time of release; and number of modified fish released.

There are still many technical hurdles to overcome before sex biasing approaches such as daughterless carp technology would ready for laboratory or field trials. Daughterless carp would initially require stocking of large numbers of genetically modified fish and would require integrated implementation with other initiatives. There are risks of public non-acceptance of intentional release of genetically modified pests into the natural environment and would therefore requires extensive public consultation.

If able to be used, effects on carp populations would be unlikely to be seen for up to 100 years due to the long generation time of carp. These pest control methods are still under development, and so feasibility and associated costs for delivery are difficult to estimate.

Cyprinid herpesvirus-3 (CyHV-3)

Cyprinid herpesvirus 3 (or CyHV-3, hereon referred to as the carp virus) can cause mass mortalities in carp under the right conditions with potential to substantially reduce wild populations, at least until resistance develops. The virus has been shown to be species-specific, and is not size specific, although juveniles are believed to be more susceptible. The strong relationship between temperature and the virus is understood to reduce in effectiveness at low or high temperatures; furthermore hybridisation between goldfish and carp could reduce the virus effectiveness. There is also the risk of public non-acceptance of the intentional release of a biocontrol agent.

As is the case with daughterless carp technology, CyHV-3 requires extensive public consultation, and further quantification of costs. Challenges may also arise as a result of resistance from stakeholders who value carp, including the ornamental koi carp industry, koi enthusiasts and commercial fishers. Furthermore, mass carp mortalities as a result of the virus release could have water quality impacts detrimental to native species. The NCCP has engaged researchers to explore these issues.

Gehrke, P. C. (2003). Preliminary assessment of oral rotenone baits for carp control in New South Wales. Managing invasive freshwater fish in New Zealand. Wellington New Zealand: Department of Conservation.

Rayner, T. S. and Creese, R. G. (2006). A review of rotenone use for the control of non-indigenous fish in Australian fresh waters, and an attempted eradication of the noxious fish, Phalloceros caudimaculatus. New Zealand Journal of Marine and Freshwater Research, 40, 477-486.

Is a virus the most effective way to control carp?

Non-scientific

Scientific

Since carp numbers exploded in Australia in the 1970’s, a variety of measures have been used to try and control carp. However, all have been unsuccessful in reducing carp impacts on a large scale. Biological control (a virus) offers some key advantages over other control approaches as it can be species-specific and highly effective when used correctly. It is also relatively cost effective.

Research into the carp virus has demonstrated that the virus is species-specific. Additionally, when conditions are right (ideal temperature, high density of carp and optimal virus concentration) the virus can result in significant carp mortality.

Additional research under the National Carp Control Plan is building on what we known about the virus, the target species (carp) and their role in Australian ecosystems. This research will be used to develop a strategy for release of the virus, which is likely to deliver optimal results for carp control and the recovery of natural ecosystems.

They can be quite species-specific when selected carefully (Suckling and Sforza 2014).

They are often capable of sustaining themselves using the population of the intended target pest species. This means although it may cost a bit to introduce a new biocontrol agent to an environment, it's a tactic that may only need to be applied once due to the self-perpetuating and self dispersing nature of biocontrol agents (Van Lenteren et al., 2003). This also means that biological control can also remain in place and effective for a much longer time than other methods of pest control. These attributes mean that biological control can be quite cost effective long terms (Saunders et al. 2010).

Most important of all, it can be highly effective when implemented on the basis of good science (Fenner and Ratcliffe 1965, Cook & Fenner 2002, Jean-Yves and Fourdrigniez 2011, Van Rensberg et al. 1987), however incomplete knowledge can lead to sub-optimal outcomes (Suckling 2013).

Carp biological control is now one of the most well researched vertebrate pest biocontrol examples worldwide. Research led by the CSIRO has shown that the carp herpesvirus is both species-specific (only affects carp) and can be highly effective on carp under the right conditions. Virus concentration, temperature and carp density appear to be critical factors in delivering a successful outcome.

Additional research now underway under the National Carp Control Plan is to building on what is known about the virus, the target species (carp) and their role in Australian ecosystems, in addition to lessons learned from previous biocontrol case studies (McColl et al. 2014) to develop a strategy for virus release which is likely to deliver optimal results for carp control, and the recovery of the health of natural ecosystems.

It is important to note that the carp virus alone will not completely eradicate carp - some carp will inevitably survive, and over time populations would rebuild. Therefore, continuing investigation of synergistic control measures, such as those that aim to alter carp reproduction biology, is important to ensure that we maximise the success of any control carp impacts.

What about sex-biasing control approaches, like daughterless carp and Trojan Y?

Non-scientific

Scientific

Sex biasing constructs such as the daughterless gene and Trojan Y can provide novel management options and even total eradication in theory, but there are many technical and societal hurdles still to be overcome before any sex biasing approach would be ready for laboratory or field trials.

Historical work on sex biasing constructs has been well supported in Australia. Effectiveness depends on many factors, including: the type of gene construct; heritability; fitness of fish carrying the sex biasing construct; size of the target species population at time of release; and number of modified fish released.

Researchers are currently considering genetic and sex biasing approaches that may complement possible use of the carp herpesvirus. Modelling conducted to date suggests that combined use of virus and genetic approaches is likely to be an optimal strategy to achieve maximum reduction in carp impacts in Australia.

Trojan Y is the easiest approach from a regulatory perspective because it is not a genetically modified organism (GMO), however the target of only producing male fish may be hard to achieve based on existing evidence of success in other species. Then, given normal Mendelian inheritance drive population change, its application requires the release of a high proportion of individuals carrying the gene-construct relative the whole population so effects may only be seen in the order of 100 years unless the carp population can first be driven to very low levels by other approaches e.g. the carp virus (Thresher et al. 2014b) due to the long generation time of carp (Thresher et al. 2014a). There would therefore be a need to stock large numbers of modified carp into natural ecosystems for many years to 'swamp' natural populations with modified carp, but this may not be as hard as it sounds given the high fertility of female carp (Thresher et al. 2014a).

The GM daughterless approach developed by CSIRO through the 1990’s to 2000’s may be more effective theoretically, because normal fertility females with the construct are less likely to have fertility issues, however the same numbers of modified carp would need to be developed for release. Stocking rivers with large numbers of genetically modified fish, may not be publicly acceptable as this would be an intentional release of genetically modified pests into natural environments.

Recent developments in gene-technology have also led to possibility of the development of synthetic gene-drive sex biasing carp constructs (Thresher et al. 2014a). This could be another form of GM daughterless, but where the all progeny of a mating between a modified carp and a wild type carp would carry the active modifier construct. Inheritance of the construct is therefore likely to be much higher than 50:50. This could in theory drive carp populations to extinction without having to swamp the feral population with carp carrying a GM construct. This approach however is still very controversial and may take many years before it could even be considered as an acceptable approach if ever (Borel, 2017).

Can't you release a predator to eat the carp?

Non-scientific

Scientific

While predators like the Murray Cod and some birds (e.g. egrets and pelicans) eat carp, they are not able to control carp impacts. Introducing a non-native predator or significantly increasing numbers of a native predator to control carp would not be wise. Unlike virus/host relationships, which can be quite species-specific, predator/prey relationships can be quite elastic. So, there is a high risk of non-carp species being preyed on by introduced predators.

It is true that predators such as Murray cod eat carp. A dietary study found 35% of Murray cod sampled (all cod >500mm total length) contained carp (Ebner, 2006). Similarly, Baumgartner (2005) found cyprinids (carp and goldfish) constituted up to 25% of prey occurrence in Murray cod sampled from the Murrumbidgee River. Though predators undoubtedly exert pressure on carp populations, they are unable to reduce carp numbers to below thresholds known to cause environmental impacts.

A number of studies have suggested that threshold densities above which carp cause ecological damage are around 100–174 kg/ha (Haas et al., 2007; Bajer et al., 2009; Matsuzaki et al., 2009), substantially lower than the historic threshold estimate of 450 kg/ha (Fletcher et al., 1985), which has formed the basis of much carp impacts research (Vilizzi et al., 2014).

Some researchers have suggested threshold densities to be even lower: Brown and Gilligan (2014) modelled that an integrated pest control approach would be required to reduce carp densities in the Lachlan River Catchment below the estimated threshold density (88kg/ha).

Introducing a novel predator to reduce carp numbers below threshold densities would not be wise.

References

Bajer, P.G., Sullivan, G. and Sorensen, P.W. (2009). Effects of a rapidly increasing population of common carp on vegetative cover and waterfowl in a recently restored Midwestern shallow lake. Hydrobiologia 632, 235–245

Baumgartner, L. J. (2005). Effects of weirs on fish movements in the Murray-Darling Basin, University of Canberra.

Brown, P. and Gilligan, D. (2014). Optimising an integrated pest-management strategy for a spatially structured population of common carp (Cyprinus carpio) using meta-population modelling. Marine and Freshwater Research 65, 538–550.

Fletcher, R.F., Morison, A.K. and Hume, D.J. (1985). Effects of carp (Cyprinus carpio L.) on communities of aquatic vegetation and turbidity of waterbodies in the lower Goulburn River basin. Australian Journal of Marine and Freshwater Research 36, 311–327.

Where did the virus come from?

Non-scientific

Scientific

The carp virus, Cyprinid herpesvirus 3, is a naturally-occurring organism first observed to kill large numbers of carp in 1997 in Germany, and then in Israel and the USA in 1998. The virus has since been detected in over 33 countries globally.

The exact origin of the carp virus has not been determined, but scientists think it most likely originated as a benign carp virus that later increased in virulence in an intensive aquaculture environment.

Interestingly, calici (the virus successfully deployed for biocontrol of rabbits in Australia) is also thought to have originated in a similar manner. Calici was first identified after it started killing rabbits in China in 1984, and was released as a biocontrol agent in Australia 11 years later. Together with other methods, biocontrol of rabbits reduced their numbers from over 10 billion to 200 million.

The carp virus is a naturally-occurring organism first observed in association with a mass carp mortality event in 1997 in Germany (Bretzinger et al., 1999).

Similar events soon followed in Israel and the USA in 1998 (Hedrick et al., 2000). The virus has since been detected in over 33 countries globally (Haenen et al., 2004; Pokorova et al., 2005; Haenen et al., 2009; OIE, 2015), with transhipment of carp by koi owners/breeders and aquaculture operators considered the most likely mechanism explaining viral spread (Gilad et al., 2003). Other vectors, such as waterbirds, cannot be ruled out, but are considered a low probability (Taylor et al., 2010, Taylor et al., 2011).

Exact origin of the carp virus has not been determined, but scientists suggest that it most likely originated as a harmless carp virus that later increased in virulence in an intensive aquaculture environment.

Interestingly, calici (the virus successfully deployed for biocontrol of rabbits in Australia) is also thought to have originated in a similar manner (Kerr et al., 2009). Calici was first identified after it started killing rabbits in China in 1984, and was released as a biocontrol agent in Australia 11 years later (Kerr et al., 2009).

Is the virus present in Australian waterways?

Non-scientific

Scientific

Data collected to date has not detected the carp virus (CyHV-3) in Australian waterways. However, additional sampling as part of the National Carp Control Plan will explore this possibility in more detail. The two most closely related viruses to CyHV-3 are reported to be present in Australian waterways. They are CyHV-1 (carp pox virus) which is carp specific, but not particularly contagious and only lethal to small juvenile fish, and CyHV-3 (goldfish hematopoietic necrosis herpesvirus) that only affects goldfish (Carassius auratus).

Data collected to date has not detected the carp virus (CyHV-3) in Australian waterways, and additional sampling under the National Carp Control Plan explore this possibility in more detail. PCR surveys of 849 carp from eastern Australia failed to detect any evidence of CyHV-3 being present in Australian carp populations (McColl and Crane (2013). This preliminary work also did not detect the presence of the two most closely related viruses to CyHV-3 : CyHV-1 (carp pox virus) which is carp specific, but not particularly virulent and only lethal to small juvenile fish, and CyHV-3 (goldfish hematopoietic necrosis herpesvirus) that only affects goldfish (Carassius auratus).

Additional sampling under the NCCP will explore whether CyHV-3 is present in Australia in more detail, and will also look for presence of other viruses that may cross react or recombine with CyHV-3 if released into Australian waterways.

How does the virus work?

Non-scientific

Scientific

The carp virus is highly contagious for carp and is mostly transferred through carp-to-carp contact.

While physical contact between infected and non-infected carp provides the most effective transmission route, carp can also become infected simply by swimming in the same waterbody as infected individuals. The carp virus also infects and kills carp most effectively within a certain temperature range (approximately 16-28°C). Further work under the National Carp Control Plan on viral transmission will inform understanding of the virus’ epidemiology.

The carp virus is present in some 33 countries worldwide. The NCCP will draw on international experience as well as the research currently being commissioned to look specifically at Australian waterways and our carp populations. The NCCP will also use mathematical modelling to inform development of a release and clean up strategy.

The virus then rapidly spreads to the kidney, spleen, fins, intestine, and brain (Gilad et al., 2004). Within the optimal temperature range, the course of infection in carp is that fish cease feeding within 3 days post exposure (dpe) and become lethargic. They then either lie at the bottom of the tank, or gather close to the water inlet or sides of the pond and gasp at the surface of the water.

Evidence of gill necrosis coupled with increased mucous secretion can present at approximately 3 dpe (Rakus et al., 2013), and these are the most consistent gross clinical signs of disease. Uncoordinated movements, erratic swimming, and twitching may occasionally be seen in very small fish (4 – 10 cm). Death occurs within 3 – 4 days after the onset of clinical signs of disease (i.e. from about 7 dpe), with most mortality occurring between 8 – 12 dpe. Loss of function in the skin, gills, kidney and gut probably account for death of the fish, but secondary bacterial, parasitic or fungal infections are also common among infected fish and often contribute to mortalities.

While transmission via water is one means through which the virus enters a carp's body, primarily via the skin and gills (Hedrick et al., 2000), direct fish to fish transmission is also possible via the skin (Costes et al., 2009). Indirect transmission due to the persistence of CyHV-3 in fish faeces (Dishon et al., 2005), plankton (Minamoto et al., 2011), freshwater mussels and crustaceans (Kielpinski et al., 2010) has also been reported. The virus may also enter via oral mucosa when fish feed on CyHV-3 infected tissue (Fournier et al., 2012).

Breeding sites and aggregations are postulated to be the primary location and time of transmission of CyHV-3 within populations (Uchii et al., 2011; Raj et al., 2011; McColl et al., 2014). Within Australia there is a steadily improving understanding of carp ecology (Brown et al., 2005; Gilligan and Asmus, 2012). In particular, the identification of discrete hotspots of carp recruitment throughout the Murray-Darling Basin offers opportunities for the targeted control of carp populations. An epidemiological model for the Lachlan River catchment has been developed that takes account of viral, host and environmental factors to inform development of a release and clean up strategy.

Improving this model’s capacity to predict viral efficacy and transmission patterns in Australian carp populations requires a more complete understanding of viral latency. Latency experiments are planned as part of the NCCP research program. It is proposed that the Lachlan model, incorporating the effects of latency, will be expanded to a Murray-Darling Basin scale under the National Carp Control Plan, to inform an implementation strategy to deliver maximum impact on carp.

Can other species transmit the carp virus?

Non-scientific

Scientific

The carp virus only infects, and affects, common carp. However, other species can carry and transmit the virus without being infected. After being “shed” by infected carp, the carp virus can survive in the water for around 3 days. However, if the virus does not infect another carp within that time, the virus will die.

While in the water, the virus may also stick to non-carp fish, sediment, plankton or other organisms/items, and may infect carp that come in contact with it. The virus does not infect these non-carp species or items - they simply carry the virus - much the same way as your dog could carry the human common cold virus on its fur if you sneezed into your hand and then patted it.

The virus only infects, and affects, common carp (McColl et al 2016, Hedrick et. al 2000). Some earlier studies have suggested other species can be infected by CyHV-3 (Davidovich et al. 2007, Bergmann et al. 2010), however did not use methods appropriate to confirm virus multiplication, and so infection.

Is the aim of the NCCP to eradicate carp?

Non-scientific

Scientific

No. It is important to note that the carp virus alone will not eradicate all carp from Australia. Australia’s experience with two other viruses that were introduced to control rabbits has reinforced that lesson. Neither the myxomatosis virus or the rabbit calicivirus could eradicate rabbits.

What viruses can do is cause a substantial drop in the numbers of their target species and reduce the ecological impacts caused by that species. Earlier studies have suggested carp start impacting on ecosystem health at densities of 100–174 kg/ha. Studies have shown that carp density is currently much higher in some areas. Biological control aims to reduce carp density below levels known to cause environmental harm.

The results of biological control can be further enhanced using additional control measures. Researchers are currently considering other (genetic) approaches that may complement possible use of the carp virus. Modelling conducted to date suggests that combined use of virus and genetic approaches is likely to be one of the best strategies to reduce carp impacts in Australia. If carp could be eliminated using an integrated approach, then the carp virus would disappear too (because the virus will only grow in carp, and will not survive in the environment for more than about 3 days). Therefore, the carp virus would only persist in Australia while carp remain a problem.

The aim of the NCCP is to develop an integrated program of measures able to reduce carp density below threshold levels known to cause environmental harm. Managing an invasive species below a density threshold, above which impacts to environmental values are unacceptable, is a key component of Integrated Pest Management (Braysher and Saunders, 2003).

A number of studies have suggested that threshold densities for carp to be 100–174 kg/ha (Haas et al. 2007; Bajer et al. 2009; Matsuzaki et al. 2009), which are much lower than historic estimates of 450 kg/ha (Fletcher et al., 1985). Some researchers have suggested threshold densities to be even lower; Brown and Gilligan (2014) modelled that an integrated pest control approach would be required to reduce carp densities in the Lachlan River Catchment below the estimated threshold density (88kg/ha).

The NCCP research program will help improve estimates of carp biomass in Australia. This will assist in considering whether reduction of carp biomass to below threshold levels is feasible. Even though available evidence suggests that the virus may be highly effective in killing carp, there will be a need to use an integrated program of measures to ensure long-term results. This will be an area of focus under the NCCP.

Smith, B. B., and Walker, K. F. (2004a). Spawning dynamics of common carp in the River Murray, South Australia, shown by macroscopic and histological staging of gonads. Journal of Fish Biology, 64, 336–354.

Won't this be like rabbits?

Non-scientific

Scientific

It's tempting to ask if viral biocontrol was used on rabbits in Australia, yet there are still rabbits around, might carp biocontrol be the same?

Rabbit biocontrol in Australia has actually been highly successful. In fact, the combination of myxoma virus and calici virus still limits rabbit numbers to about 15% of their potential numbers, and without them the cost to agriculture alone would be in excess of $2 billion per year.

Just like rabbits, carp biocontrol is unlikely to ever eradicate every carp. However, it is possible the carp virus may help to reduce carp density below levels known to cause environmental harm, which is the aim of the NCCP. Just like farmers are enjoying the benefits of reduced rabbit numbers, so too will communities benefit from healthier and cleaner waterways.

Earlier studies have suggested that carp start impacting on ecosystem health at densities of 100–174 kg/ha. Numerous studies have shown that in places, carp density is currently much higher.

The NCCP research program will help improve estimates of carp biomass in Australia. This will assist in evaluating feasibility of reducing carp biomass to below threshold levels known to result in ecological impacts.

Rabbit biocontrol is perhaps the most successful example of vertebrate pest biocontrol worldwide. The combination of myxoma virus and rabbit haemorrhagic disease virus still limits rabbit numbers to about 15% of their potential numbers, and without them the cost for agriculture alone would be in excess of $2 billion per year.

The cumulative environmental benefits of the release of myxoma virus (MV) in 1950 and rabbit haemorrhagic disease virus (RHDV) in 1995 includes landscape scale native vegetation regeneration, increased abundance of native plants and animals, continued persistence of many native threatened species, large scale carbon biosequestration, and improved landscape and ecosystem resilience. The cumulative economic benefits for agriculture alone from MV and RHDV over 60 years are estimated at $70 billion, or an average of $1.17 billion per year.

Just like rabbits, carp biocontrol is unlikely to ever kill the last carp. However, it is possible that the carp virus may help to reduce carp density below levels known to cause environmental harm, which is the actual aim of the NCCP.

Earlier studies have suggested that carp start impacting on ecosystem health at densities of 100–174 kg/ha. Numerous studies have shown that in places, carp density is currently much higher.

The NCCP research program will help improve estimates of carp biomass in Australia. This will assist in evaluating feasibility of reducing carp biomass to below threshold levels known to result in ecological impacts.

Won't this be like cane toads?

Non-scientific

Scientific

The introduction of cane toads (Rhinella marinus) to Australia in the 1930s is one of the most notable examples of poor early biological control practice.

Introduced before Australia had mature biosecurity legislation or environmental risk assessment based regulations for importing exotic organisms, the cane toad was released based on overseas commentary and without any direct assessment of its likely effectiveness as a biocontrol agent on the targeted cane beetle.

After release, it quickly became apparent that the toxic toads did not effectively prey on the cane beetle, but were devastatingly efficient in preying Australian native species. Free from natural predators, and with abundant food supplies, toad numbers and distribution quickly exploded, creating the ecological disaster we see today. Today, Australia’s biosecurity regulatory environment is world class and nothing like this could legally happen again.

Under these regulations, if carp biocontrol is used, it will be informed by rigorous planning and world-class risk assessment processes based on robust evidence to indicate carp biocontrol can be done safely and effectively.

The introduction of cane toads (Rhinella marinus) to Australia in the 1930s is one of the most notable examples of early poor biological control practice.

Introduced before Australia had any mature biosecurity legislation, or environmental risk assessment based regulations for importing exotic organisms, it was released based on overseas commentary and without direct assessment of its likely effectiveness as a biocontrol agent on the targeted cane beetle.

After release it quickly came to light that the toxic toads did not effectively prey on the cane beetle, though were devastatingly efficient in preying on a diversity of Australian native species. Free from natural predators, and with abundant food supplies, toad numbers and distribution quickly exploded, creating the ecological disaster we see today. The Australian biosecurity regulatory environment is now world class and nothing similar could legally happen again.

Under these regulations, if carp biocontrol is used it will be informed by robust planning, and careful risk assessment processes based on robust evidence to indicate carp biocontrol can be done safely and effectively.

Australia's National Carp Control Plan is operating in a completely different era in which the efficacy and host specificity of the carp virus on common carp been extensively studied and peer reviewed. Research conducted to date demonstrates that common carp present in Australia are highly vulnerable to the carp virus, that the virus only causes disease in European carp (also known as common carp), and that all other species are not susceptible.

Effectiveness on European carp

Research proposed under the National Carp Control Plan will further build our understanding of possible risks and benefits of carp biocontrol. A thorough, systematic quantitative assessment of social, economic and ecological risks is proposed, in addition to a robust and transparent process for quantifying expected benefits and costs of carp biocontrol in Australia.

Research conducted to date demonstrates that the carp virus is specific to common carp and its ornamental variety, koi carp (Hedrick et al., 2000), although susceptibility varies across carp strains (OIE, 2014). A broad range of other fish species has been tested with varying degrees of rigour; none developed the disease (Bretzinger et al., 1999; Perelberg et al., 2003; Haenen et al., 2004; Haenen and Hedrick, 2006; Uchii et al., 2009; OIE, 2015). Furthermore, no CyHV-3-associated mortalities have been reported in any fish or other animal anywhere in the world (Gotesman et al., 2013, Michel et al., 2010, OIE, 2012).

The CSIRO-Australian Animal Health Laboratory has conducted susceptibility trials using bath and intra-peritoneal injection methods alongside carp controls (McColl et al., 2016). The species selected represent a broad range of taxa with a breadth of evolutionary relationships with the order Cypriniformes, including:

Research conducted to date also demonstrates that European carp present in Australia are highly vulnerable to the carp virus, that the virus only causes disease in European carp, and that all other species are not susceptible.

Research proposed under the National Carp Control Plan will further build our understanding of possible risks and benefits of carp biocontrol. A thorough, systematic quantitative assessment of social, economic and ecological risks is proposed, in addition to a robust and transparent process for quantifying expected benefits and costs of carp biocontrol in Australia.

The carp virus will also not be released until it has been independently evaluated by regulators in two government departments against two regulated review processes and assessed under the Biological Control Act and approved by Chief Veterinary Officers of all jurisdictions

What will everything eat when carp numbers are significantly reduced?

Non-scientific

Scientific

Some Australian native species including Murray cod, Eastern water rats, cormorants and long-nosed fur seal prey on the pest fish species carp. Consequently, there is a need to understand what might happen if carp numbers are significantly reduced following possible implementation of the National Carp Control Plan.

Though there are only few studies which have examined the importance of carp as a food source in Australian ecosystems, available research suggests that they do not comprise a large part of the diet of most species. This is partly because juvenile carp often live in different habitat types to predators such as adult Murray cod. Also, carp quickly grow to a size that is too large for most predatory species to eat.

A recent Queensland study demonstrated that reducing carp numbers can cause subsequent explosions in biomass levels of other native prey items including zooplankton and small-bodied native fish. This work indicates that reducing carp numbers may increase food available for predatory species, not decrease it. This, in turn, may result in healthier populations of species which eat these prey items, including popular native angling species.

Carp make up a significant proportion of the biomass of fish in many Australian rivers (Harris et al., 1997; SRA Unpublished Data; Lintermans, 2007), and potentially significant reductions in carp abundance and biomass levels if the National Carp Control Plan is implemented have prompted some to ask the question "what will species that currently prey on carp eat?". The insinuation is that without carp, these species might not have sufficient food available and starve. But what does the available research say?

Whilst there is not a significant body of research available on the contribution that carp make to the diet of native Australian species, what information does exist suggests that they generally do not form a significant dietary component. In fact, Koehn et al. (2004) suggests the rapid expansion of carp within Australia may have been assisted by lack of predatory pressure. This is largely because the rapid growth rate of carp enables them to quickly reach a size that precludes their consumption by most predators.

Nevertheless, some species of native fish, waterbirds, and charismatic fauna do prey on carp to some extent. In particular, Australia’s largest predatory freshwater fish, the Murray cod, has been shown to predate upon carp. A dietary study by (Ebner, 2006) found 35% of Murray cod sampled (all cod >500mm total length) contained carp. Similarly, Baumgartner (2005) found Cyprinidae spp. (carp and Goldfish) constitute up to 25% of Murray cod prey occurrence in cod sampled from the Murrumbidgee River.

Examination of Murray cod stomachs from Rivers of the Southern Murray Darling Basin found <7% of Murray cod stomachs sampled contained carp (Doyle et al., 2012). Carp up to 410 mm total length have been recorded in stomach of large Murray cod. Doyle et al. (2012) attributes the low occurrence of carp in the diet of Murray cod to the variation in the habitat utilisation between early life stages of carp, which primarily inhabit shallow floodplain-type habitats, and Murray cod that prefer main channel habitats.

Golden perch and Australian bass also consume small carp, though infrequently, and each species would be incapable of consuming adult carp due to their gape size limitations (Ebner, 2006, Doyle et al., 2012). The critically endangered Trout cod has also been shown to predate carp, however carp made up <1% of their prey (Baumgartner, 2005).

Terrestrial vertebrates that have been shown to predate upon carp and other Cyprinids include feral cats (Jones and Coman, 1981), the Eastern water rat (Woollard et al., 1978) and cormorants (Miller, 1979), however in each instance European carp constitute a small proportion of the diet. Hughes et al. (1983) suggests carp within billabongs may provide a reliable food source for Australian Pelicans. Koehn et al. (2004) suggests with few effective predators, sequestered detrital carbon, rather than passing up through subsequent trophic levels of macroinvertebrates and smaller fish (Bunn and Davies, 1999), may become ‘locked’ away from the trophic chain for the lifespan of a carp (up to 50 years) (Bănărescu and Coad, 1991).

So some native Australian species do prey on carp. However, it is important to note that native species which currently prey on this pest species have not always relied upon them for food. In years gone by when carp were absent or in much lower numbers in Australian waterways native prey items including zooplankton, invertebrates, small fish, biofilms were more abundant. Ebner et al. (2006) suggested major shifts in prey availability have influenced the ecology of Murray cod and the structure and function of the food web in the rivers of the Murray-Darling Basin.

The decreased diversity of native prey species provides opportunity for Murray cod to exert a larger per capita effect on carp (Pimm, 1982, Ebner, 2006). However in microcosm trials both Murray cod and golden perch consumed carp relatively infrequently compared to native prey species (Doyle et al., 2012). There are very few published studies which provide an insight into potential alterations to food web dynamics that may result from significant reductions in carp biomass within Australian aquatic ecosystems.

Prey switching of carp’s dominant predator, the Murray cod, may exert increased pressure upon other native species and decapods, though the ecosystem will eventually reach a predator/prey equilibrium (McColl et al., 2014). Furthermore, the reduction of carp may allow the proliferation of native prey items of predatory species including Murray cod returning the ecosystem closer to its former state before the proliferation of carp in the 1960s. Indeed the findings of Gehrke et al. (2010) showed that after a significant reduction in carp biomass within several experimental wetlands the biomass of small-bodied native fish increased by up to three times the biomass of carp removed. In the Queensland study, carp biomass was reduced within two of four lagoons, removing 43% and 33% of carp biomass, 34 and 26 kg per hectare respectively. The other two lagoons remained untouched, for comparison. In the two lagoons where carp were controlled, native fish biomass increased by 90 kg per hectare, roughly three times the biomass of carp removed. Added to this, large zooplankton populations (e.g. Boekella and Daphnia) increased 10 times and populations of aquatic insects and crustaceans also boomed. In the two lagoons where nothing was done, populations of native fish, zooplankton, aquatic insects or crustaceans did not change.

What this work suggests is that native fish are much more efficient in their use of food resources than carp (producing three times the biomass) and that removing carp will likely increase the food available for predatory fish and waterbirds, not decrease it. This should ultimately lead to bigger, healthier populations of popular native angling species and waterbirds.

Don't Murray Cod rely on carp for food?

Non-scientific

Scientific

While Carp currently make up around 25-35% of the diet of Murray cod, research has shown that cod prefer native prey items if given a choice. If carp numbers are reduced, Murray cod will switch foods. This behaviour is common in many fish species.

Studies have also shown that a reduction of carp can allow small native fish, and the microscopic food they eat, to flourish. This makes the food web healthier and more natural.

One study showed that native bony bream biomass increased 240% and 1130% after carp reduction in two experimental lagoons, while native gudgeon biomass increased by more than 1600% in one lagoon. Lagoons where no carp control occurred showed no increase in native fish over the same period.

Australia’s largest predatory freshwater fish, the Murray cod, has been shown to eat carp. A dietary study by (Ebner, 2006) found 35% of Murray cod sampled (all cod >500 mm total length) contained carp. Similarly, Baumgartner (2005) found cyprinids (carp and goldfish) constituted up to 25% of prey occurrence in Murray cod sampled from the Murrumbidgee River.

Examination of Murray cod stomachs from rivers of the Southern Murray-Darling Basin found <7% of Murray cod stomachs sampled contained carp (Doyle et al., 2012). Doyle et al. (2012) attributes this low occurrence of carp in the diet of Murray cod to differences in habitat utilisation between early life stages of carp that primarily inhabit shallow floodplain-type habitats, and Murray cod that prefer main channel habitats. Given the large gape of Murray cod and the species’ ability to attain up to 180cm in length (Lintermans, 2007), large Murray cod are likely to be the only fish predator of adult carp. Carp up to 410 mm total length have been recorded in stomachs of large Murray cod (J. Stocks, pers. comm.). Golden perch and Australian bass may also consume small carp, though these species would be incapable of consuming adult carp due to their gape size limitations (Ebner, 2006; Doyle et al., 2012). The critically endangered trout cod has also been shown to eat carp, but carp made up <1% of their prey (Baumgartner, 2005).

Australian freshwater ecosystems are now significantly modified, with native fish biomass significantly reduced, and carp are now the dominant species within many Australian rivers (Harris and Gehrke, 1997; SRA unpublished data; Lintermans, 2007). Ebner et al. (2006) suggests these major shifts in prey availability have influenced the ecology of Murray cod and the structure and function of the food web in the rivers of the Murray-Darling Basin. The decreased diversity of native prey species provides opportunity for Murray cod to exert a larger per capita effect on carp (Pimm, 1982; Ebner, 2006). However, in microcosm trials, both Murray cod and golden perch consumed carp relatively infrequently compared to native prey species (Doyle et al., 2012).

Fish regularly shift diet in response to varying availability of food resources (Werner and Gilliam, 1984). If carp numbers were greatly reduced, prey switching of carp’s dominant predator, the Murray cod, may exert increased pressure upon other native species and decapods, though the ecosystem should eventually reach a predator/prey equilibrium (McColl et al., 2014). Further, the reduction of carp may allow the proliferation of native prey items of Murray cod, thus returning the ecosystem closer to its former state before the proliferation of carp in the 1960’s. Indeed the findings of Gehrke et al. (2010) showed that after a significant reduction in carp biomass within several experimental wetlands the biomass of small-bodied native fish increased by up to three times the biomass of carp removed. Bony bream (Nematalosa erebi) biomass increased by 240% and 1130% in the two experimental lagoons, while gudgeon (Hypseleotris spp.) biomass increased by more than 1600% in one lagoon. Native fish in control lagoons showed no increase over the same period. Similarly, anecdotal evidence from western New South Wales suggests that macroinvertebrates may increase in abundance following carp reduction (Ellis, 2016).

Koehn et al. (2004) suggests that, with few effective predators of carp in the ecosystem, detrital carbon that is sequestered in the bodies of carp, rather than passing up through subsequent trophic levels of macroinvertebrates and smaller fish (Bunn and Davies, 1999), may become ‘locked away’ from the trophic chain for the lifespan of a carp (up to 50 years) (Bănărescu and Coad, 1991). Equally, significant reduction in carp numbers within Australian rivers may enable nutrients to once again become available to Australian native species. It should be noted, however, that other invasive species (redfin perch, goldfish, oriental weather loach) may also benefit from reduced carp numbers.

Will the virus affect other species?

Non-scientific

Scientific

Research conducted by CSIRO scientists over the last decade tells us, no, the virus remains specific to Common carp only. However, other species, like goldfish, may carry virus on them without infection for short periods of time. These are known as passive carriers.

It's a bit like how your dog could carry the human cold virus particles around on its fur if you sneezed on it, but it would not actually be sick with the cold.

Virus particles floating free in water can attach to passive carriers, like non-carp fish and other aquatic organisms. The virus does not multiply within the cells of these passive carriers, and so these carriers are not infected. The period for which virus particles can adhere to the body of a passive carrier is likely to be up to three days.

A broad range of native fish species and other animals have been tested with varying degrees of rigour, and none developed the disease. Further, no CyHV-3-associated mortalities have been reported in any fish or other animal anywhere in the world.

It is correct that levels of mortality were observed in some native species tested, howeverall native fish deaths in CSIRO experiments were proven (in the peer-reviewed literature) to not be caused by the carp virus. Fish die in captivity for a multitude of reasons, and particularly when taken from the wild and/or transported to new facilities, so a level of mortality was not unexpected. The crucial thing was to determine whether any deaths were due to infection with the carp virus, or CyHV-3.

In order to detect an ‘infected’ fish, it is essential to use a method called RT-PCR, which can confirm virus replication. And even then, it is important to use suitably-designed primers (a primer is a short nucleic acid sequence that provides a starting point for DNA synthesis). No studies have detected replication of the carp virus in species other than common carp when using appropriate methods and primers.

In addition to the peer-review process that this work has undergone during the publication process, we have commissioned an additional independent review of non-target susceptibility research conducted by CSIRO under the NCCP.

Studies indicate that CyHV-3 is specific to Cyprinus carpio. This species represents all wild, farmed and ornamental common carp and koi carp present in Australia. Hybrids of koi x goldfish and koi x crucian carp are also affected by CyHV-3 disease, with mortality rates ranging from 35% to 91% (Bergman et al., 2010). Common carp x goldfish hybrids have been reported to show some susceptibility to CyHV-3 infection, but the mortality rate observed was rather limited (5%) (Hedrick et al., 2006).

Researchers from the Centre for Environment, Fisheries and Aquaculture Science (CEFAS) in the United Kingdom conducted transmission studies with various cyprinid species and found that only common carp are affected; they also tested goldfish, tench and orfe, all three of which are in the same taxonomic order (Cyprinidae) as carp (Haenen and Hedrick, 2006). Bretzinger et al. (1999) noted that sturgeon (Acipenser species; order Chondrostei) and goldfish (Carassius auratus; Order Cypriniformes) remained unaffected when held with affected Koi, while Perelberg et al. (2003) found that tilapia (Oreochromis niloticus; Order Perciformes), American silver perch (Bairdiella chrysoura; Order Perciformes), silver carp (Hypophthalmichthys molitrix; Order Cypriniformes), goldfish (C. auratus; Order Cypriniformes), and grass carp (Ctenopharyngodon idella; Order Cypriniformes) were all resistant to disease after exposure to sick fish. Furthermore, they also found that even after cohabitation with infected carp, fingerlings of these species did not transmit disease to susceptible carp. Species whose susceptibility was being examined were exposed to virus by co-habitation with infected carp, and direct inoculation was not used.

Yuasa et al. (2012) developed an RT-PCR (based on the terminase gene) for CyHV-3. It is possible to have 2 primers for the RT-PCR, each completely in adjacent exons (with an intron in between). This then gives two products, depending on whether the primers are annealing to genomic DNA or mRNA. Alternatively, one of the primers can span the exon-exon boundary (when the intron has been excised in the formation of mRNA; also known as the splice junction). If used at a high annealing temperature, this primer would not work on genomic DNA. This was the method used by Yuasa et al. (2012)

Yuasa et al. (2013) subsequently provided the most compelling evidence that goldfish are not infected by CyHV-3. Firstly, they bath-inoculated goldfish with CyHV-3, but could demonstrate no infection. Then, they bath-inoculated goldfish with the virus, and then washed them twice in virus-free water. These goldfish were then exposed to susceptible carp, either for one day at 0-1 days post exposure (dpe) of the goldfish, or from 2-24 dpe of the goldfish. Some mortality was observed in the former group of carp, but none in the latter group. This suggested that mortality of carp was probably due to mechanical transfer of virus from the first group of goldfish, but, by 2-24 dpe all virus attached to goldfish had lost infectivity. The implication is that goldfish are not infected by CyHV-3; rather, any virus originating from goldfish had simply been attached to the goldfish.

It was the RT-PCR designed by Yuasa et al. (2012) that was subsequently used by McColl et al. (2016) in their study of non-target species susceptibility to CyHV-3. McColl et al. (2016) conducted susceptibility trials using bath and intra-peritoneal inoculation methods alongside carp controls. The species selected represent a broad range of taxa with a breadth of evolutionary relationships with the order Cypriniformes, including:

Native catfish are the closest Australian native fish relatives to carp. Does this mean that catfish could be vulnerable to the virus?

Non-scientific

Scientific

No. While the group of fish species that catfish belong to is the most closely related to the common carp group, catfish and carp are not closely related at all. This means that catfish species native to Australia are not susceptible to the carp virus. CSIRO research has confirmed that two species of Australian native catfish were not infected, or affected, by the carp virus.

The most closely related taxonomic order to the one carp belongs to is Siluriformes (Catfish), leading some to question whether their level of relatedness is sufficient to create risk of species jump.

To understand this question, a brief journey into the science of phylogenetics is necessary. Phylogenetics investigates the evolutionary origins and relationships of organisms at varying levels of taxonomic organisation. Thus, phylogenetic ‘relatedness’ among organisms is defined by how recently (in evolutionary terms) two or more groups diverged from a common ancestor and emerged as taxonomic entities recognisable in their modern forms.

Carp and catfish belong to two distinct taxonomic orders, Cypriniformes and Siluriformes respectively. Cypriniformes and Siluriformes form part of a broader phylogenetic grouping of fishes called the Otophysi. The two other otophysian orders are Gymnotiformes (electric eels and American knifefishes), and Characiformes, which includes piranhas and tetras.

Otophysi forms one of two series in the superorder Ostariophysi. A superorder is a fine-scale taxonomic ranking above Order and below Class (see below), and encompasses greater technical detail than is required for a general understanding of the evolutionary relationship between carp and catfish.

The Otophysi are generally considered to be monophyletic, meaning that, in the distant evolutionary past, they arose from a single common ancestor before diverging into different species and moving across the Earth’s surface to occupy their present geographical ranges (Briggs, 2005. Otophysian evolution has been debated, and is complicated by a span of more than 100 million years for which no fossil evidence has been discovered (Briggs, 2005; Santini et al., 2009; Nakatani et al., 2011; Chen et al., 2013).

Nonetheless, molecular evidence suggests that Cypriniformes emerged as a recognisable taxonomic entity between 130 and 186 million years ago, while Siluriformes diverged somewhat earlier, between 162 and 198 million years ago (Nakatani et al., 2011). The fossil record provides relatively more recent divergence dates, with the oldest-known cypriniform fossil being about 61 million years old and the oldest siluriform fossil being between 83.5 and 88.6 million years old (Chen et al., 2013).

Divergence inferences from the fossil record can only be based on available fossils and cannot account for undiscovered material or species that no left fossilised remains. Consequently, divergence times estimated from the fossil record are usually underestimated (Anderson, 2012). The most recent common ancestor of carp and catfish therefore lived tens, and perhaps hundreds, of millions of years ago. Thus, catfish are indeed the Australian native fishes most closely related to carp, but this not imply that the evolutionary relationship is particularly close.

Broad description of taxonomic relatedness

The degree of taxonomic relatedness between carp and catfish may be contextualised by a general overview of the taxonomic hierarchy (the scheme scientists use to classify living things). From broadest (i.e. least related) to narrowest (i.e. an individual species), the taxonomic hierarchy is:

Kingdom: Kingdom is the broadest level of biological classification. For example, all multicellular animals, whether mosquitos or elephants, are classified into the Kingdom Animalia. Taxonomists in Australia, Great Britain, and several other countries generally recognise five kingdoms; Animalia, Plantae, Fungi, Protista, and Monera, while American taxonomists sometimes divide the Kingdom Monera (bacteria) into two kingdoms (Archaeabacteria and Eubacteria), making a total of six kingdoms.

Phylum: The Phylum is another very broad level of classification. For example, the Phylum Chordata includes all vertebrates and some invertebrates. The defining features of chordates (animals within the Phylum Chordata) are possession, at some stage in the life-history, of:

Pharyngeal slits

A dorsal nerve chord

A notochord

A post-anal tail

Thus, human beings (and indeed all mammals), birds, reptiles, amphibians, fish, and some invertebrates, such as sea squirts, all belong in the Phylum Chordata.

Class: There are approximately 107 animal classes, although this number can vary based on taxonomic revisions. As an example of the degree of relatedness implied by this taxonomic rank, human beings belong in the class Mammalia, along with all other mammals (i.e. whales, seals, dolphins, cats, dogs, horses, cows, mice etc).

Focussing specifically on fish, the class Actinopterygii, to which carp and catfish both belong, includes all fishes apart from sharks, rays, and jawless fishes (lampreys, hagfish). Thus, for example, carp, catfish, barramundi, all tunas, marlin, mullet, gudgeons, gobies, coral trout, Murray cod, and approximately 24,000 other fish species are all actinopterygians.

Order: Order is the taxonomic rank at which carp (order Cypriniformes) and catfish (order Siluriformes) diverge. To place the concept of order in context, human beings belong in the order Primates, along with lemurs, lorises, tarsiers, monkeys, and apes. Domestic dogs belong in the order Carnivora, along with cats (including the big cats), seals, walruses, weasels, skunks, hyaenas, and many other predatory mammals.

Family: At this taxonomic rank, carp and catfish have now diverged. The order Cypriniformes is divided into 11-12 families, with common carp (Cyprinus carpio) belonging to family Cyprinidae. The order Siluriformes, to which catfish belong, contains more than 30 families. The two catfish species tested for susceptibility to Cyprinid herpesvirus 3 belong to two separate families, Ariidae (forktail catfishes) and Plotosidae (eel-tailed catfishes).

Genus: Organisms sharing this taxonomic rank are closely-related. Introduced common carp are the only species from the genus Cyprinus occurring in Australia. Human beings belong to the genus Homo. Modern humans (i.e. us) are the only extant (surviving) representatives of this genus.

Species: Individuals within a species can interbreed to produce fertile offspring. The common carp is a species; Cyprinus carpio, while modern humans are also a species; Homo sapiens. A scientific name for a species thus comprises the genus name (e.g. Cyprinus), which is shared by all species within that genus, and the specific epithet (e.g. carpio). Taxonomists are sometimes interested in defining sub-species, a finer classification again, but this level of detail is unnecessary for the present discussion.

To summarise, carp and catfish are as closely related as Humans and Lemurs (that is to say, they are not particularly closely related).

References

Anderson, J.S. (2012). Fossils, molecules, divergence times, and the origin of Salamandroidea. Proceedings of the National Academy of Sciences, 109, http://www.pnas.org/cgi/doi/10.1073/pnas.1202491109

Can you explain mortality of native fish in CSIRO experiments?

Non-scientific

Scientific

Please see the scientific answer to this question for detailed information.

All of the results for the work conducted by CSIRO on the susceptibility of non-target species (NTS) to the carp herpesvirus have been published in the peer-reviewed ‘Journal of Fish Diseases’ (2017) 40:1141-1153 (attached). Table 1 in this scientific paper gives the full mortality results from the susceptibility trials.

As has been noted, levels of mortality (sometimes quite high) were, indeed, observed in some native species that were tested. However, as stated in the paper:

"In 11 cases where NTS were challenged with virus, no mortalities were recorded (Table 1). In a further 10 cases, mortalities in negative control groups matched, or exceeded, those in viral-challenged counterparts, suggesting that CyHV-3 was not affecting these NTS."

Mortalities in negative-control fish were clearly not due to virus because at no stage were these fish exposed to the carp herpesvirus. So, mortalities in negative-control fish suggested that other factors could account for the losses, not only of negative-control fish, but also of fish in the virus-challenged groups.

What could these ‘other factors’ be? Stress associated with bringing wild fish into a very unfamiliar environment would likely be an important factor. The effects of existing parasite burdens in many wild-caught fish species would also have been exacerbated by the stress of captivity. In addition, all experiments were conducted at water temperatures of 21–23 °C. While this is the optimal temperature for virus activity, it is not necessarily the optimal temperature for all fish species that were examined. And finally, the methods used to challenge fish with virus were undoubtedly stressful.

All fish species were examined by bath exposure to the carp herpesvirus (simulating the potential natural route of infection). This involves fish swimming in a high-protein, aqueous solution that contains high levels of virus (or, for negative controls, the same solution but without virus). While the protein solution is not toxic, it probably does interfere to some extent with oxygen exchange across the gills. Hence, many fish species may be stressed by this process, some more than others, and this probably accounts for some subsequent mortalities (in virus-challenged, and negative-control groups).

In many cases, fish were also challenged by direct inoculation of the virus into the fish, or, for negative-controls, inoculation of an equivalent volume of an innocuous fluid. This process is known as ‘intraperitoneal inoculation’. While this procedure was only conducted on relatively robust species of fish, the anaesthesia and handling involved in this process would also no doubt stress many fish.

In other words, in these 4 instances, mortality in virus-challenged fish was greater than in the negative controls. However, when the cause of this mortality was investigated further, there was no evidence to incriminate the carp herpesvirus. Firstly, most of the mortalities occurred far too early, or too late, in the course of infection to be consistent with the carp herpesvirus (Figure 1 in the journal paper). Secondly, using highly sensitive and specific molecular tests (a combination of a PCR test, and an RT-PCR test with specially-designed primers), there was no evidence that the carp herpesvirus was present, or had multiplied, in any of the fish (and, without virus there can be no disease attributed to the virus).Finally, microscopic examination of tissues from dying fish failed to reveal any changes that would be consistent with a viral infection.

So, the conclusion for these three fish species was that, yes, a proportion of the fish had died during the challenge trial, but “while it is not clear what caused the mortality in these animals, it was clearly not due to (the carp herpesvirus)”.

Was CSIRO testing contaminated?

Non-scientific

Scientific

Genetic testing methods are well-recognised for a propensity for occasional contamination problems due to their extreme sensitivity.

The CSIRO scientists who did the study said they did, indeed, experience some contamination problems with their PCR (yielding so-called ‘false-positive’ results on occasion), but, through the use of the RT-PCR [and advanced molecular technique], were able to provide complete clarification of the ‘false-positive’ results. Certainly, the professional referees at the ‘Journal of Fish Diseases’ were happy with their work, and found no faults with it.

See the non-scientific answer to this question.

Why weren't all Australian native species (& sizes) tested?

Non-scientific

Scientific

Representatives of almost all major groups of freshwater fish that carp interact with across their distribution in Australia have been tested and shown no evidence of infection or mortality caused by the carp virus (see the list of species tested and see figure below). The remaining few, like salamanderfish from W.A., are being tested by CSIRO under the NCCP research program.

Clearly, it is not possible to test the susceptibility of every native fish species in Australia to the carp herpesvirus (there are too many species, and not sufficient time or resources to test them all). So, the approach the CSIRO researchers adopted was to test the susceptibility of representative species from each of the 10 taxonomic groups of Australian freshwater, or estuarine, fish that could conceivably come into contact with an infected carp (if it were to be released in Australia).

There are only a few more species of native fish that remain to be tested, like salamanderfish from W.A.. These are currently being tested by CSIRO under the NCCP research program.

The overriding aim with the susceptibility trials was to create conditions that would give the carp herpesvirus the best chance to cause disease in native fish species. If, in the face of such favourable conditions, the virus failed to infect or produce disease, then its safety could be assured.

For this reason, small (usually immature) fish were purposely chosen for the challenge trials because it is these fish that have the most immature immune system. If any fish was going to be susceptible to the carp herpesvirus, then it would be those with immature, poorly-developed immune systems. For all fish viruses, larger and older fish invariably are more resistant to infection and/or disease.

In addition, while an important ‘law’ of biology is that experimental animals should be stressed as little as possible, it was felt that, in this case, the normal stresses of captivity (see earlier) would likely increase any potential susceptibility of native fish to the carp herpesvirus. So, again, the researchers were (in some cases, inadvertently) creating conditions that would give the virus the best chance to cause disease.

Despite all these conditions, the carp herpesvirus was unable to multiply or cause disease in any of the native fish species that were tested.

Note: Larvae of native fish species were purposely not tested for susceptibility because work with the carp herpesvirus in carp (the host species of the virus) has shown that larvae under 1 cm are, in fact, not susceptible to infection (see Ito et al, 2007).

Do genetic changes in the virus pose an unacceptable risk?

Non-scientific

Scientific

There is a low risk of the carp virus undergoing genetic changes in such a way as to result in a potentially increased risk to Australian native species. The carp virus is a DNA virus, which tend to be quite stable, increasingly so for those with larger genomes. The carp virus is one of the largest, and therefore most stable, viruses from its scientific family.

Of course, it is important to recognise that all viruses mutate and carp herpesvirus, myxomatosis virus, calicivirus are no exception, but mutation does not directly relate to increased risk. Mutations very rarely equate to dangerous events such as host jumps, or increased virulence and the timeframes of such rare events are more easily quantified in terms of millions of years!

That lesson comes from long term evolutionary studies and extensive observations of viruses in the wild: the carp herpesvirus has been widely distributed in the world for over 20 years, the myxomatosis virus has been in Australia for over 60 years, and the calicivirus for over 20 years.

Researchers have mapped how each of these viruses have undergone many mutations in the wild during those periods, but despite these time-spans, there is no evidence that any of these viruses have ever expanded their host range or indeed jumped into another species.

Research at the University of Sydney has also shown that, even for those very rare events when viruses do jump species, invariably the jump is into a very closely-related species. Carp belong to a group of fish known as ‘cyprinids’, and it is important to note that there are no native cyprinids in Australia. So, the chance of a host-jump by the carp herpesvirus in Australia would be vanishingly small (simply because there are no closely-related native cyprinid species).

At approximately 295,000 base pairs, CyHV-3 has the largest genome in the family Alloherpesviridae (Fournier and Vanderplasschen, 2011; Davison et al., 2013). This high stability is supported by the high similarity of all CyHV-3 isolates at the sequence level (>99%).

Further evidence in support of this assessment of low risk includes the lack of reports that CyHV-3 has extended its host range beyond common and koi carp in those countries where it is endemic since first recognized in the mid-1990s (Aoki et al., 2007, Hedrick et al., 2000), and a lack of evidence that closely related cyprinid herpesviruses, CyHV-1 (Sano et al., 1985) and CyHV-2 (Jung and Miyazaki, 1995) have extended their host range since discovery.

Can you 100% guarantee the virus will not mutate if released?

Non-scientific

Scientific

Science can never help us to guarantee anything 100% – including gravity holding us to the earth.

However, science can help us to determine the probability of something occurring. And there is a low probability of the carp virus mutating or otherwise evolving in ways that would enable it to infect a new host species (including Australian native species). This low risk is partly because the Carp virus is a type of virus – a double-stranded DNA virus - for which mutation of a type and magnitude that would enable infection of a new host species is not a primary mechanism of viral evolution, and partly because common carp are not closely related to any Australian native species.

Because nothing in science is 100% guaranteed, the possibility of the carp virus eventually evolving a new host association cannot be excluded, however such a process would operate over longer timescales than this control strategy would require if approved.

100% certainty doesn’t exist in real science.

Science seeks evidence to support or refute a hypothesis (or some other scientific principle like a theory). It’s all about the evidence (and the quality thereof), not about proving with 100% certainty.

Consequently, it is not possible to ever be 100% sure of anything – including gravity. We can, however, determine whether something has a low probability of occurring. And there is a low probability that CyHV-3 will mutate once released and resulting in a potentially increased risk to non-target species.

This is because the carp virus is a DNA virus, which tend to be quite stable, increasingly so for those with larger genomes. At approximately 295,000 base pairs, CyHV-3 has the largest genome in the family Alloherpesviridae (Fournier and Vanderplasschen, 2011; Davison et al., 2013). This high stability is supported by the high similarity of all CyHV-3 isolates at the sequence level (>99%).

Additional evidence in support of this assessment includes the lack of reports that CyHV-3 has extended its host range beyond common and koi carp in those countries where it is endemic since first recognized in the mid-1990s (Aoki et al., 2007, Hedrick et al., 2000), and a lack of evidence that closely related cyprinid herpesviruses, CyHV-1 (Sano et al., 1985) and CyHV-2 (Jung and Miyazaki, 1995) have extended their host range since discovery.

Because nothing in science is 100% guaranteed the possibility of the carp virus eventually evolving a new host association cannot be excluded, however the ability of herpesviruses to jump hosts has been inferred using phylogenetic approaches considering evolutionary events over millions of years (Geoghegan et al. (2017), not the decades expected of this control strategy.

Informing possible release

When and where might the virus be released?

Non-scientific

Scientific

The carp virus will not be released before the end of 2018.

During 2017, the NCCP will embark on a large program of research and consultation. The two key components of this program will be: a series of scientific projects conducted by independent researchers at Australian universities; and, a series of community engagement forums (i.e., town hall events) across areas affected by carp.

At the end of 2018, the NCCP will make a formal recommendation on the best way to control carp impacts in Australia. This recommendation will be a document called 'The National Carp Control Plan'. It will be based on the results of the research projects funded by the NCCP and the input from communities during the consultation process.

If it is recommended that the carp virus form part of a suite of carp-control measures, and formal approval is granted, the carp virus may then be released. In that case, the initial release sites and specific pattern of release would follow the results of relevant research funded under Research Theme 3: Informing possible implementation.

Could the virus be released in a controlled manner (i.e. staging to reduce the impact of a clean-up)?

Non-scientific

Scientific

This is being investigated by the National Carp Control Plan. Scientists and planners are currently working on how to release the virus in the most effective way to reduce carp populations, but also to manage risks and impacts.

Scientists are building their understanding of how the virus is likely to impact carp populations and waterways through a series of research projects funded by the NCCP, and previous research undertaken worldwide.

This research is revealing:

that the effectiveness of the virus is strongly influenced by water temperature and closecontact with infected carp;

where significant carp biomass is located in our waterways; and,

the conditions and thresholds at which dead carp will affect water quality.

This knowledge will allow more accurate predictions about how the virus is likely to work in specific waterways, and gives authorities a more precise and controlled way of reducing carp populations, while being able to manage risks.

Planners are also playing a role in building more precision into how the virus could be released by mapping virus management within discrete carp control zones. These are areas of catchments and waterways bounded by significant barriers to upstream fish passage (river regulating structures or natural barriers). These zones can then contain the impacts of the virus and allow a staged release.

To control the risks of downstream, unplanned virus spread (resulting from high water flows or floods) the virus release can start from downstream carp control zones and then move to upstream carp control zones.

A staged release of the virus within discrete control zones would also allow for the efficient use of management resources. Activities such as clean up could be more focussed, rather than being spread over numerous locations at the same time.

Successful control of a pest species over a large geographic range can be logistically challenging, and this is certainly true of carp control in Australia. Common carp are now present in every Australian state and territory with the exception of the Northern Territory, constituting up to 80% of fish biomass in some river systems (Harris and Gehrke, 1997, Sustainable Rivers Audit unpublished data). Identification of a strategy for staged release would help reduce logistical challenges, and this is an area of focus under the NCCP.

Possible phasing of a virus release is not without challenges. In particular, effectively compartmentalising such a large, geographically, climatically, and hydrologically diverse landscape poses numerous challenges. The world’s largest rat extermination program on South Georgia Island offers some useful clues for success (Poncet et al., 2011). The aim of this program was to eradicate brown rat (Rattus norvegicus) from a 170 km long, 10 - 40 km wide sub-Antarctic island 1400 km east of the Falkland Islands through introduction of 183 tonnes of poison over 224 square miles (580 km2). Through robust metapopulation research of the target species (Robertson and Gemmell, 2004) it was learned that the island’s unique climate and topographical attributes resulted in several isolated rat populations separated by large glaciers.

This knowledge enabled development of a staged program for implementation, in which the island was divided into a number of treatment zones, which were treated individually. Using this staged, methodical strategy the project team were able to progressively move across the island, treating each zone and testing effectiveness before moving on until eventually the entire island was treated successfully.

This is a noteworthy accomplishment clearly considered impossible by some in 1980 (Pye and Bonner, 1980), who reported brown rats to be “an established part of the wildlife of South Georgia”, and also reported that “no management procedures would be possible to reduce or control the existing rat population even if this were thought desirable”. This successful outcome, delivered despite earlier pessimism, highlights the value of adopting an evidence-based strategy, coupled with a ‘can do attitude’ when tackling significant pest control challenges.

While there is no evidence of genetic structuring of carp in Australia, the discontinuous nature of many Australian waterways (including the Murray-Darling Basin) resulting from extensive installation of flow regulating infrastructure may offer a means via which the release of CyHV-3, and subsequent clean-up of carp biomass, may be logically phased. Under the National Carp Control Plan it is proposed to explore opportunities to utilise barriers to fish migration to separate waters into discrete treatment zones, enabling carp to be treated within each zone in a staged manner. Implementing a controlled release strategy using barriers to fish migration would require identifying in-stream structures impervious to drown-out in all but major floods, and timing release and clean-up to avoid significant flooding.

What will happen to the dead carp?

Non-scientific

Scientific

Dead carp will occur in waterways where the virus is released. A critical challenge for the NCCP is to demonstrate how we can manage dead carp in a way which avoids impacts on water quality, people, livestock and native species. To meet this challenge, the NCCP is undertaking research, and talking to experts and local communities about how to respond to dead carp biomass.

The response ideas that researchers, stakeholders and experts come up with will be tested and refined through regional case studies which will workshop how the virus release can be managed in a specific region. The case studies will involve all the relevant authorities, stakeholders who might be impacted and people with local knowledge about carp and their waterways.

The NCCP has already identified a range of methods to respond to the build-up of dead carp at a wide range of locations including:

regulating water flows to flush, move or dry out water bodies where dead carp biomass is located

The specific chosen methods for managing dead carp will depend on local conditions and arrangements.

Dead carp that are removed from the waterways will be transported to regional processing facilities wherever possible. The NCCP has a specific research project which will recommend how the dead carp biomass can be used. Where is it not possible to use the dead carp, they will be disposed of at approved waste disposal sites.

Virus response will be managed through co-ordinated regional, state and national bodies that bring together relevant government agencies and local authorities. The community and commercial sector will also be involved in the response to the virus release.

Like any worthwhile endeavour, carp biocontrol presents some challenges. Maintaining water quality for use by people, stock, and native species is one such challenge. The NCCP recognises the importance of this task, and our approach to developing a practical, effective, and flexible clean-up strategies is outlined below.

Intuitively, we can all understand that major carp mortality events entail some risks to water quality. However, understanding the exact nature and magnitude of these risk may require a specialised approach. Research commissioned under the NCCP will include a scientific risk assessment quantifying risks associated with the proposed carp biocontrol program, including the clean-up. Hayes et al. (2007) provide an overview of the methods used in scientific risk assessment.

Biomass estimates: how many carp are there, and where are they?

Successful clean-up requires understanding carp abundance and distribution at several spatial scales, from continental through to particular habitat types. The NCCP will commission a multi-method biomass study, providing the most accurate picture ever developed of carp distribution and abundance in Australia. Methods used will include:

capture-recapture studies

acoustic and radio-tagging

collation and statistical interrogation of all pre-existing carp abundance datasets

physical measurement of carp biomass when lakes and wetlands are drained as part of ecological remediation works

environmental DNA (e-DNA, a suite of methods that enable detection of a species and estimation of its abundance based on DNA shed into the water)

This multi-method approach will enable cross-checking and triangulation, enhancing the accuracy and rigour of resulting biomass estimates.

An ecosystem perspective on clean-up requirements

Planning the clean-up requires knowledge of the virus’s behavior in wild carp populations, including seasonal patterns of viral latency and re-emergence (Eide et al., 2011; Xu et al., 2013). To enable this understanding, an epidemiological model of the carp virus’s behaviour across all 29 river catchments of the Murray-Darling Basin will be developed. The model will identify optimal seasons, locations, and release strategies for the virus, and in so doing will also pinpoint times and places where carp mortality events are likely, allowing for response planning.

Hydrological models, developed and tested over many years, will also examine the effects of varying levels of carp biomass on dissolved oxygen levels in a range of aquatic habitat types. Mosley et al. (2012) provide an example of a similar modelling process. These models will be complemented by detailed experimental studies in real ecosystems (see Boros et al., (2014) for an example of this kind of experiment). Additional research may also explore nutrient interception pathways in freshwater ecosystems, identifying options for avoiding blue-green algae blooms. Together, these research projects will enable response planning that safeguards water quality. For further reading in these areas, Brookes et al. (2005) discuss nutrient interception pathways, while Carmichael and Boyer (2016) review health impacts of blue-green algae.

How to eat an elephant: compartmentalising clean-up

Successful control of a pest species over a large geographic range can be logistically challenging, and this is certainly true of carp control in Australia. Common carp are now present in every Australian state and territory except the Northern Territory, making up more than 80% of fish biomass in some river systems, and up to 93% in some areas (Harris and Gehrke, 1997). Logistically, it would impractical to seek to employ a simultaneous pest control strategy for common carp across the species’ distribution; a phased approach is required.

The need to phase any release and clean up strategy also presents some clear challenges. In particular, how to compartmentalise such a large, geographically, climatically and hydrologically diverse landscape. The world’s largest rat extermination program in South Georgia offers some useful insights here. The aim of this program was to eradicate brown rat (Rattus norvegicus) from a 170km long, 10-40km wide sub-Antarctic island 1400km east of the Falkland Islands through introduction of 183 tonnes of poison over 224 square miles (580km2). Through robust metapopulation research of the target species (Robertson and Gemmell, 2004) it was learned that the island’s unique climate and topographical attributes resulted in several isolated rat populations separated by large glaciers. This knowledge enabled development of a staged program for implementation, in which the island was divided into a number of treatment zones, which were treated individually (Figure 1). Using this staged, methodical strategy the project team were able to progressively move across the island, treating rats in each zone, testing effectiveness in each zones before moving on until eventually the entire island was treated successfully. This is a noteworthy accomplishment clearly considered impossible by some in 1980 (Poncet et al., 1980), who reported brown rats to be “an established part of the wildlife of South Georgia”, and also reported that “no management procedures would be possible to reduce or control the existing rat population even if this were thought desirable”. The successful outcome delivered in spite of earlier pessimism highlights the value of adopting an evidence-based strategy, coupled with a ‘can do attitude’ when tackling significant pest control challenges.

Figure 1. Glaciers enable South Georgia to be divided into discrete zones for the purpose of rodent control (Figure reproduced from Poncet and Poncet, 2009).

While there is no evidence of genetic structuring of carp in Australia, the discontinuous nature of Australia’s Murray-Darling Basin resulting from extensive installation of flow regulating infrastructure may offer a means via which the release of CyHV-3, and subsequent clean-up of carp biomass may be logically compartmentalised and phased (see Figure 2). Under the National Carp Control Plan opportunities are being explored to utilise these assets to separate waters into discrete treatment zones, enabling carp to be treated within each zone in a staged manner.

Figure 2. Dams and weirs present throughout the Murray-Darling Basin (in green). Opportunities will be explored to use these compartmentalise reaches into zones, enabling progressive treatment for the control of carp (source: Murray-Darling Basin Authority)

Using flow

Many Australian rivers are highly regulated, with locks, weirs, and dams controlling water movement (Growns, 2008). The NCCP is working with river managers to identify ways that flows can be manipulated to assist release and clean-up and maintain water quality.

Boots on the ground: the logistics

Results from these research projects will show us what needs to be done. Expert help will then be enlisted to work out how we do it. The NCCP will form a Critical Issue Advisory Group composed of experts from areas including military and transport logistics, commercial carp harvesting, and large-scale human- and animal-health responses. These experts will develop detailed strategies for rapidly responding to carp mortality events, including identifying equipment and personnel needs.

What risk do dead carp pose to water quality - in particular dissolved oxygen and blue-green algae?

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Scientific

Live carp reduce water quality by stirring up mud as they feed and excreting nutrients into surrounding waters, which in turn promote blue-green algae blooms. Dead carp can also have impacts if left in a waterway. In sufficient numbers, dead carp can reduce oxygen levels in water, and as they decompose they can release nutrients into surrounding water, causing algal blooms and changes in water chemistry.

Research proposed under the National Carp Control Plan will help to improve current understanding of how different quantities of dead carp impact on water quality in the variety of habitats that carp inhabit in Australia. Information collected will inform risk assessment and development of clean-up methods.

Studies show that living carp muddy waters, increase nutrient levels (thereby promoting blue-green algae blooms), and reduce abundance of water plants (macrophytes), invertebrates (e.g. aquatic insects and crustaceans), and some fish species (Vilizzi et al., 2014, 2015; Weber and Brown, 2009). For example, Weber and Brown (2009) found that carp increased water turbidity (muddiness) in 91% of surveyed studies, reduced invertebrates in 94%, and reduced macrophytes in 96% of surveyed studies (Weber and Brown, 2009). A more recent meta-analysis supported these results, finding strong evidence for carp impacts on all the same ecosystem components (Vilizzi et al., 2015).

Dead carp can also have impacts if left in a waterway. In sufficient numbers. Large organic matter inputs - including from dead fish - can result in low dissolved oxygen concentrations as microbes consume oxygen during respiration while decomposing the organic matter (King et al. 2012). This is particularly true in shallow habitats with high carp biomass levels, as well as in high temperature or thermally-stratified environments (Brookes, unpublished data, J. Marshall, pers. comm.).

Large organic matter inputs are usually associated with flood events, during which vegetation litter is inundated and dissolved organic carbon (DOC) is leached from the substrate (Whitworth et al. 2014). While the effects of vegetation derived DOC on DO concentrations have been well documented in the MDB (Gehrke et al., 1993; McMaster and Bond 2008), the effects of a large organic matter input in the form of fish carcasses on dissolved oxygen have been poorly understood to date.

Research proposed under the National Carp Control Plan will help to improve current understanding of how different quantities of dead carp impact on water quality in the variety of habitats that carp inhabit in Australia. Information collected will inform risk assessment and development of clean up methods.

McMaster, D. and Bond, N. (2008). A field and experimental study on the tolerances of fish to Eucalyptus camaldulensis leachate and low dissolved oxygen concentrations. Marine and Freshwater Research, 59, 177-185.

Will 2,000,000 tonnes of carp die in 2 days?

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Scientific

No. The total amount of carp present in Australia, distribution of that biomass, and likely spread of the carp virus if released are all key areas of focus for research under the National Carp Control Plan.

Options for phasing possible virus release will also be investigated under the NCCP research program, including use of barriers to fish migration (dams and weirs) to break large waterways into smaller units. If possible, this would enable carp to be treated within each discrete stretch of waterway in a staged manner.

The NCCP is a process, not a forgone conclusion. Should the findings from the NCCP research suggest that the release of a virus to control carp is the recommended approach, we will also be ensuring we make recommendations – based on research and consultation with a vast number of stakeholders – as to how this virus should be released and managed to ensure a successful outcome for all.

No. However the total amount of carp present in Australia, distribution of that biomass, and likely spread of the Carp virus if released are all key areas of focus under the National Carp Control Plan.

Options for phasing possible virus release will also be investigated under the NCCP research program, including use of barriers to fish migration (dams and weirs) to break large waterways into smaller units. If possible, this would enable carp to be treated within each discrete stretch of waterway in a staged manner.

If infected by the virus, could carp change their behaviour to reduce mortality and impact the overall effectiveness of the control program?

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Scientific

While possible, research being undertaken as part of the National Carp Control Plan will allow this risk to be better understood. A previous study (Rakus et al., 2017) reported that carp infected with the carp virus appeared to change behaviour in lab trials and would congregate around heating elements. On this basis, it was thought that carp may seek out warm water refuges within Australian waterways, reducing the overall program effectiveness.

Researchers are developing models to understand Australian waterways including water quality, flow and connectivity, how carp live and behave in those waterways, and how the carp virus impacts on carp populations. As part of this work, researchers will also consider the impact of any behavioural change in carp during infection, including identifying the presence of warm water refuges, where they might occur and the role they may play in reducing overall effectiveness of the program.

Rakus et al. (2017) reported that carp infected with the carp virus appeared to change behaviour in lab trials, and would congregate around heating elements in trials. On this basis they postulated that carp may seek out warmwater refuges within Australian waterways, in doing so reducing overall program effectiveness. This is a possibility, however is also being studied under the NCCP to enable this risk to be better understood.

Researchers under the NCCP are conducting epidemiological modelling to better understand patterns of viral transmission, spread, and mortality. Epidemiological knowledge will also be required to predict the locations and environmental conditions in which major carp mortalities are likely, and equally, areas where sub-optimal outcomes may be experienced. The predictive capacity necessary for planning both release and clean-up will be provided primarily by the NCCP’s epidemiological modelling project.

The project uses coupled epidemiological, hydrological, and ecological models to investigate the carp virus’s likely behaviour in Australian ecosystems. The epidemiological modelling team is currently focussing on the crucial, and inter-related, roles that water temperature, carp physiological condition (especially spawning-related stress), and carp density are likely to play in transmission and mortality. To maximise the model’s predictive capacity, final modelling will be based on empirical water temperature, biomass, and virus transmission data.

Would the possible release of the carp virus produce ongoing mass fish kills?

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Scientific

Research undertaken throughout the world tells us this is unlikely to occur. The carp virus is now found in 33 countries worldwide. It has not caused major ongoing fish kills in any of these countries, although recurrent kills in Lake Havasu, Arizona, have been anecdotally attributed to the carp virus. Researchers investigating presence of the carp virus in Japan sampled 109 natural waterways. Not one experienced ongoing outbreaks.

However, the National Carp Control Plan is committed to making its recommendations and decisions based on research undertaken in our own waterways. Some of Australia’s leading researchers are working collaboratively to explore the possible release of the carp virus in our waterways and to ensure we have a clear and accurate understanding of its benefits and impacts. In particular, CSIRO scientists are currently working closely with the NCCP to model the epidemiology of the virus to predict carp population responses to different release scenarios, including likelihood of repeat outbreaks.

The carp virus is now found in 33 countries worldwide, in both natural ecosystems and man-made water bodies/fish farms. Initial disease outbreaks in natural ecosystems can be significant, however there are very few examples of repeat outbreaks in a specific waterbody. Those few that have been reported point to low level mortalities following large initial carp kill events.

Taylor et al. (2010) also observed that clinical outbreaks rarely recur in the same sites in years following the initial outbreak in UK fisheries. The few waterbodies where repeat outbreaks have been reported are largely artificial water bodies stocked with carp for recreational fishing, or Koi collections.

Research that will be conducted under the National Carp Control Plan will further contribute to our understanding in this area. Epidemiological modelling will be used to predict carp population responses to different release scenarios, including likelihood of repeat outbreaks.

Are ongoing outbreaks and low oxygen events inevitable?

The carp virus has now been found in 33 countries worldwide, and has not caused ongoing mass fish kills in any other country, suggesting the likelihood of ongoing outbreaks occurring is low.

However, computer modelling undertaken under the NCCP research program may suggest there is value in re-treating specific areas (for example, nursery habitats) with the carp virus following initial release to deliver optimal results. Research proposed over the next 18 months will add to current understanding in this area.

As for hypoxia or anoxia events (low/no oxygen levels in water), the risk of these occurring due to carp biocontrol is being investigated under one of the research projects as part of the National Carp Control Plan. Both hypoxia and anoxia already occur within waterways as oxygen levels are dynamic and change with things like wind flow, velocity and high dissolved organic carbon.

The research is aiming to predict the impact of carp mortality on the dissolved oxygen concentration of wetlands, rivers and floodplain habitats and whether water flow management can mitigate the risk.

International examples suggest that it is unlikely that largescale ongoing outbreaks and anoxia events events would result. A study of prevalence of CyHV-3 in Japanese rivers reported that of 109 rivers sampled, no repeat outbreaks were reported both before and after sampling took place in any rivers studied (Minamoto et al., 2012).

Taylor et al. (2010) also report that clinical outbreaks were rarely reported at the same site in years following an initial outbreak in UK waterways. The few waterbodies where repeat outbreaks have been reported are largely artificially stocked water bodies for recreational fishing, or Koi populations.

How do we know that carp won’t just develop immunity and rebuild?

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Scientific

Based on lessons learnt from past use of viral biocontrol agents for invasive vertebrates, and on mathematical modeling, the carp virus will likely have the greatest impact in the first few years after release. After that, effectiveness may be diminished - but not lost - as virus and host adapt to each other. Earlier modeling suggested that carp populations may recover to 30 - 40% of present levels within a decade of virus release.

The release would therefore need to be complemented by secondary control measures to ensure enduring results. Genetic strategies are being carefully considered under the NCCP to work synergistically with the carp virus. These strategies would skew the sex ratio of the remaining carp population after release of the virus. As abundance of one sex diminished, so too would the whole population. New generations of more virulent, but still natural, strains of the carp virus may also be investigated.

Of course, any strategy to manage carp impacts will be most effective if supported by efforts to promote ecosystem recovery through habitat restoration, native fish restocking, restoring native fish migration pathways, and addressing water quality concerns.

Community

What is the National Carp Control Plan?

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Scientific

The Fisheries Research and Development Corporation (FRDC) is leading a $15 million planning process, on behalf of the Australian Government, to inform development of an integrated strategy for the control of carp impacts in Australia, built around a backbone of biological control.

The National Carp Control Plan (NCCP) aims to help recover the health of Australian waterways and aquatic biodiversity. The NCCP will be based on thorough and measured approaches, ensuring the benefits and risks of carp biocontrol are understood and the right recommendations are made to government in 2018 to ensure optimum outcomes for Australia.

What work is the NCCP undertaking?

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Scientific

The team working on the National Carp Control Plan are delivering a comprehensive program of research, and consulting extensively with stakeholders and the community to inform development of an integrated plan to control carp in Australia.

Why isn't the NCCP catching carp right now?

The NCCP is charged with developing a plan to reduce carp impacts at the national scale. Specifically, to assess the potential use of biocontrol.

Local reductions in carp numbers are great, but have no real meaningful impact on river health at the continental scale. That's why we're focussing on developing a plan for Australia.

Will the virus affect humans?

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Scientific

Three primary lines of evidence enable us to be very confident that the carp virus won’t affect humans.

First, a report to the European Commission by the Scientific Committee on Animal Health and Welfare found that there is no evidence of ANY fish virus ever being transmitted to humans.

Second, researchers have attempted to culture the carp virus on human cell lines, and cell lines of other primates (i.e. apes and monkeys) without success. In other words, even deliberate and concentrated attempts to infect human and other primate cells with the carp virus have been unsuccessful.

Third, the virus’s history in carp aquaculture globally provides a practical demonstration of the carp virus’s inability to infect humans. People in thirty-three countries where carp virus is present have been repeatedly exposed to the carp virus without a single documented case of infection by the virus. This exposure has included clean-up from virus outbreaks, when workers have close, repeated contact with carp that are shedding large quantities of the virus. In addition infected fish are regularly eaten and the absence of infections under these conditions provides confidence that the virus is not transmissible to humans.

There are multiple lines of evidence demonstrating that CyHV-3 will not infect humans:

The virus has been described since the 1990’s and is now present in over thirty-three countries. Fishers, aquaculturists and Koi enthusiasts come into contact with the virus on a regular basis through interaction with water and/or fish carrying virus particles, and no adverse effects have been documented.

A significant though unquantified proportion of carp sold internationally for human consumption are vaccinated with a weakened strain of the virus, and no human health concerns have been raised in relation to consumption. Israel alone produces 5 – 6,000 tonnes of carp per annum for human consumption, of which the majority is vaccinated (Pers Comm. Arnon Dishon.)

Carp aquaculturists in some countries harvest farmed carp immediately upon observing clinical signs of CyHV-3 and sell infected fish at a reduced price (Pers. Comm. Ayi Santika). Despite this no human health concerns have been raised in relation to human consumption.

McColl et al. (2016) have tested mice as a model mammal species and confirmed that the virus did not replicate within inoculated mice.

Dishon (2007) attempted to infect cell cultures of homoeothermic, mostly mammalian origin both at 37°C and 22°C which did not result in either cytopathic effect or presence of virus by PCR. Cells included embryonic chick cells (CEF), XC (a rat cell line), HeLa (human cell line) and CV-1 (monkey origin cell line). Importantly, there has been no evidence of any fish virus causing disease in humans (European-Commission, 2000).

All fishers, aquaculturists, researchers, fisheries managers and community groups surveyed from Indonesia, the United States, United Kingdom, Israel and Japan during a recent international study tour confirmed that they have never experienced any health issues, including respiratory, skin, eye or oral irritation/sensitisation as a result of contact with the virus in either its wild or attenuated form.

Can't we just eat them all?

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Scientific

Apart from the fact that some Australian states prohibit the possession of carp, there has historically been relatively little interest in the species as a table-fish in Australia. However, there is no doubt that carp are seen as a useful food source by other nations, particularly those suffering from poor food security. We are investigating options for the wise use of carp biomass, irrespective of the control method used.

Can't we have a big carp muster?

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Scientific

Carp musters are great fun, and help to engage communities in pest control, however research demonstrates that angling events only reduce populations by approximately 0.5% - 1.8% which unfortunately is not enough to reduce ecosystem impacts caused by carp.

Carp musters are great fun, and help to engage communities in pest control, however research demonstrates that this will not result in a lasting reduction in carp numbers (Norris et al., 2013).

For example, Norris et al., (2013) reported carp angling to be the least effective of all harvesting methods examined, with population reductions ranging from 0.5%–1.8% across angling competitions examined. Norris et al., (2007) also estimated population reduction by anglers in the Goondiwindi Carp Cull to be 0.5% compared to 13.4% for electrofishing (Norris et al., 2007). Similarly, in 2008, anglers in the Goondiwindi Carp Cull removed 40 carp from Rainbow Lagoon, equivalent to 1.9% of the estimated population, and much lower than the catches provided by other methods (Norris et al., 2013).

Models presented by Thresher (1997) and Brown and Walker (2004) demonstrate that unless carp populations can be a reduced by a large percentage, physical removal is unlikely to offer an effective method for carp control. On this basis, Gehrke et al. (2010) suggest that low-cost carp angling events provide an effective method for promoting community awareness of issues surrounding carp in the Murray-Darling Basin, but their effectiveness in reducing carp populations and environmental impacts is low.

Can't the dead carp be used as fertilizer?

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Scientific

Carp are currently used to make fertilizer in Australia , and it may be possible to do so on a larger scale if the carp virus is used to reduce carp numbers and impacts.

Researchers are exploring options for utilisation of carp biomass under the National Carp Control Plan. A wide range of options are being considered, including composting, conversion into fishmeal, and fertiliser.

Carp are worth money. Can't we sell them?

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Scientific

It is true that carp are worth money, however the concept of selling carp for profit is more complex than it sounds. Carp are one of the most farmed and consumed freshwater fish species worldwide. Because of this, the average global price of Carp is very low: approximately $1.50 per kilo. This makes it extremely difficult to harvest, chill and export carp profitably.

How can the community get involved?

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Scientific

During the two years of the NCCP’s development, the NCCP project team will speak to stakeholders and visit regional centres to provide updates on the progress of the plan and gather community feedback.

The project team wants to understand your local waterways, what's important about them and how you use them, and your concerns and questions so that they can be addressed in the plan.

The NCCP team has been meeting regularly with communities members and interested groups across the carp distribution areas and we will be conducting an initial round of regional workshops and public meetings across the seven participating states and territories. The regional workshops/public meetings will be held in appropriate locations within the natural resource management areas likely to have an interest in this project.

These public information sessions will be held in the evening and give community members the opportunity to hear first-hand from the project team about the background, context and desired outcomes of the NCCP as well as the proposed approach towards its development. It is important these public information sessions position the impact of carp as a ‘whole of community’ issue and, as such, encourage all members of the community to opt in to the discussion. Importantly we will want to hear directly from community members about what is important to them.

Why isn't the NCCP holding a community forum in my town?

We simply don't have the resources to go to every town, but we are doing our best to consult with as many people as possible. There is a raft of ways you can have your say on the NCCP.

The NCCP has partnered with natural resource management groups in each state and territory where carp exist to identify areas most affected by carp and the potential use of biocontrol, and focus on those. The current community consultation forums (running from October 2017 to February 2018) are the first round of events, with another round of community consultation forums to occur from March-November 2018.

The NCCP has also embarked on a major research project to survey Australian’s attitudes and opinions on carp and the NCCP, with a series of surveys to take place over the next 12 months or so.

In additional we are also seeking direct feedback from individual people – nationally - by phoning or emailing us. You can also find out more about where the NCCP is up to by registering for our newsletter.

Remember you can still have your say by emailingcarp@frdc.com.auor calling 1800 CARP PLAN (1800 2277 7526).

How can I keep up to date on the latest info?

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Scientific

Register your details at the bottom of our homepage to keep up to date on when we will be in your region.

Contact the National Carp Control Plan team via the contact us page or call 1800 CARPPLAN.